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 ...