I ask why politely and then secretly laugh with my friends behind their back when they can't provide criticism and hem and haw because they didn't expect to get this far. Tactic never fails.
I simply think that the story was not created for them. Take a look around. Even the wildest story finds its readers.
I'm really dyslexic so if they are talking about bad grammar/awkward sentences, I would have preferred if they would have addressed it in that way. Saying "Your writing is trash." doesn't help me. Maybe if their profile doesn't look like an edgelord, I might ask why because I am open to fixing those mistakes. I know there are people who just have no tact and are not trying to be trolls, they feel they have to put in their two cents.
If they said "Your story is trash", I would be confused. My stuff might be boring or confusing to some but I don't understand why their word of choice would be trash.
Would probably agree with them, to some extent and then tried to defend my story a little, I had this happen with my sister and it took a week of debating to finally agree XD. I'm happy we did cause I went and fixed it and now I'm happier with the result
Ignore them if they're not trying to be constructive.
If they are being constructive take notes and try to improve after getting some other form of feedback here and there.
I wouldn't really say anything as I dislike confrontation. I would probably believe them and possibly even thank them for being honest, then just never talk to them again. (It depends on their relationship with me).
I kind of take comments with a grain of salt because I think if someone is trying to communicate something meaningful they will usually do it in a space where other people aren’t going to engage and upvote and receive attention for posting their opinion. That doesn’t necessarily make it invalid but people can and do opt for more private means of communication like DMs or email when it comes to matters that could be sensitive.
But if someone is just trying to push my buttons I’d ignore it. Those people want you to engage and react in one way or another and if you just refuse to give them the time of day they’ll usually just go away eventually.
"Aren´t we all trash in some way?"
No, but seriously, either I ignore the comment or just give the "K" reply.
Kill them with kindness. I try to be very sweet and polite, and sometimes that just riles the person up more, which is a good indicator they're more interested in trying to get a rise out of you than give real feedback.
I generally might respond with something along the lines of "I'm sorry you feel that way/I hope you find something more to your taste." Though I don't mind engaging in the criticism if the other party is actually interested in talking and not just talking crap.
If it's a person I know in real life I'm WAY more sensitive though. I'd probably cry? 
I'd pity them and just smile. What else is there to be done?
Every story has his flaws. And every story has its good points. There are ways to express one's opinions in a constructive manner, in a manner that would permit for the author to elevate his work to the next level.
However, using words with negative connotations, such as trash, is certainly not one such manner. In fact, using such words generally denotes unhappiness, irritability. It implies an emotional response to something. That something may not be an author's story. In fact, it most probably isn't. When such words are used, there is a inherent desire to hurt the receiving person. Insults do not come from a happy place. Misery likes company. Unhappy people sometimes want to make others unhappy too.
The truth is, when I consider a work unworthy of my time (gosh, this sounds mean), I do not go to the trouble of commenting at all, I simply banish it from my thoughts and move on. But someone was so irked, triggered, as we like to say, by something about that author's behavior, story or by nothing at all, really, that that someone actually took the time, even if it is 10 seconds of his life, to a write a mean comment. Those are 10 seconds of his life he will never get back.
Damn, I am sorry for that person just thinking about it ...
Honestly I would get very offended hahaha. but I would act like I don't care, after I would get sooo angry 
, then I would be all worried, "what if they are right?"
and finally I would cry and feel terrible. I am very sensitive 
But I would keep writing and drawing, what else can I do! It's what I love!!
It really depend whether the person criticizing ACTUALLY said the story is trash, or if the person receiving the criticism INTERPRETED as 'this person says my story is trash', because the criticism is harsh.
First case: I would have pretty much no reaction. I don't care. 'Not constructive: next!'. I may not be very confident in my beginner comic skills, but there is no way there is nothing more to say than 'it's trash'. I think any work where the creator put some effort worth... more effort than 'it's trash' as a critic.
Second case: I personally love harsh constructive criticism. Also love nice constructive criticism. As long as it does the job of making me think, evolve and reconsider, I love it.
But I understand sometimes, people need things to be said more nicely. So I would say, if someone is in this situation of receiving very harsh, but constructive criticism, and is upset and depressed by it: remember that for some people, criticizing harshly means they think you have what it takes to be much better; that they have faith in you and think you can improve significantly. Now, obviously, that is not what's behind all harsh constructive criticism. But I think it worth remembering, that it's not necessarily an aggression. Sometimes, ot's a compliment.
If it was online, I'd probably ignore it. If it happened IRL, I'd probably laugh awkwardly and leave.
People leave comments like that to get a rise out of people--if you respond, it gives their argument validity, it gives them a rush, it encourages their bad and childish behavior. So, the best response, IMO is no response and a firm block.
If it's just constructive criticism I might not block but I probably won't bother responding. Too many people get waaay too wrapped up in webcomics and expect them to be this high art when it's like...yo I'm making this for free. You get what you get.
But tbh all of my comments on my comic have been very nice. I think mean comments is mostly a problem for people who have a very large following.
I really dont have time to waste on opinionated arguments. If someone were to say it, and their opinion doesnt matter to me, I basically shrug & keep it moving. In this day & age there are too many folks who think their opinion holds some type of weight or value to it; even more so when they arent even a student in the respective field. Then there are others who think that for a work to be "to a standard" it has to have "this element" and/or "that element" and if it doesnt have these things then the work isnt "to standard"- that is bullshit as well...
well, some of my reader said i need to improve my grammar and it helped me a lot so i often do a improvise to my chapter.
right now im working on a Light Novel and i enjoy it so much.
critiques is needed even sometime deep in your heart is hur.t
If they can't muster to tell why exactly they think my story is trash, i would roll on the floor laughing. Too many people these days think they're entitled to an opinion while forgetting what it takes to form one.
If they break down why my story is trash, i'd feel a bit sad.
Tbh, I know my writing is trash. That's why I'm making an experimental comic and posting it on line for free. I'm just learning. If I was great at writing and art, I'd probably try and sell it or something.
So, I'd probably just say 'Thanks for pointing out the obvious! What do you think about my art though?'
If someone said my art or character designs were trash, that's what would upset me more. I know I've got a looonnggg way to go in my artistic journey, but drawing really is my passion, so I'd be upset that they couldn't actually say something constructive, or see any value in my art at all.
I'd thank them. I'd make an effort to not sound too sarcastic, though I might throw in a, "Well, bless your heart."
I might ask for suggestions on how I should write, and ask them to cite a specific area that needs improvement. If they can't do that, reframing their trash review as a constructive critique, I just shrug it off. Maybe I'd silently stew for a bit about useless criticism.