I would do nothing since a predictable twist is better than destroying the story just to "subvert expectations" *looks at the fire from GoT
Do nothing. Banning them, erasing comments, and gas lighting them will just make you look like an asshole.
I had a short comic which someone guess the twist at the end and I was sort of happy because it meant they were paying attention. Having set ups to your plot twist is important, even if it is a surprise, there should be evidence to back up why it happens. I was able to guess the plot twist in the infamous film The Boy because there is a scene where the MC knocks on the wall and it's hallow. This is also why I think the plot twist in Frozen isn't good.
Easiest thing to do is nothing, let it feel like a theory to them. If you're like me, and can't help but respond to every reader comment, you might say something like "Ohh, that's interesting!" or "I like your theory, commenter!". Readers are smart cookies, and part of the fun of creating is seeing if one or two get close to guessing your plot. At least to me anyway ^^;
(Sorry, Kiweevil, meant to reply to the topic!)
Why should I care? Let them guess right , that 's the whole fun of being a reader in the first place. Most theories are stuff they want to happen and if it turns out to be true then you have made a well constructed plot for people who pay attention to detail.
However if everyone is guessing it like it was the most obvious thing, maybe just throw in a red herring here and there or play some mind games but overall it doesn't really matter its just a theory. Banning and deleting only confirms their theory and if very very dickish, punishing readers who pay attention and are invested and discourages both. I think conforming directly will ruin the fun a bit in my opinion but really if your audience is big enough you will always get one person saying your plot twist in a sea of far of guess to very close guesses.
Thats good, it means that your plot is so smart that only a single person was able to correctly figure it out, on the other hand tough, if everybody is able to guess the twist it means that you did something wrong
I feel like there's a difference between a spoiler and a fan theory that happens to be right? Because like, for the former, that's someone straight up telling you what's going to happen, and that's pretty annoying. But for the latter, usually anyone who's invested enough in the story to guess the plot twist ahead of time will have more of a reaction of, "AHA! I correctly put together all the clues! I'm so smart!", like they just solved a mystery. At least that's my experience with it.
But yeah, my general response to fan theories, correct or not, is "hm yes, that sure is an interesting observation" 
If a commenter guesses the plot twist, good for them. It means they were paying attention to the bread crumb trail. It's kind of fun to see people speculate and speculate rightly about what is coming down the line.
I might joke with them about it? Probably wouldn't confirm or deny outright. A good plot twist usually drops hints and it's not a bad thing to know some people are enjoying the story casually and will get a surprise while others are happily connecting the dots as you draw them out. Just let readers read, that's kind've their thing .
Honestly? I would feel very relieved... It's the "oh, thank goodness, SOMEONE can see what this story is about. It's not too confusing after all" kinda thing.
The appeal of my story doesn't come from plot twists, anyway. My most recent chapter, for example, gave out some semi-unexpected information, but its most important impact comes from delivering what the readers were supposed to expect/dread.
I'd do nothing, lol. If someone guessed the big plot twist, this means they're invested in the story. Also good for them for being smart. 
I at most do nothing, people can go with their theories and stuff if they want. There was one comment I got on webtoons in my second chapter.. but I didn't give a response to it
Usually my comments are talking of current situations, compliment of art or some friendly critique on ensuring the pages are readable. I do get some interesting ones to where I can't give a response XD more like.. "Is this meme...? Am I getting this old to not know???" and I have gotten one question or two that seemed.. rhetorical??
kind of hard to read sarcasm but overall, it's up to you to give the response or just quietly turn your head away and continue the ride. As my boss says, you are the driver. They're the ones riding
i actually like to encourage them without directly confirming anything. honestly it can be really rewarding to figure out a plot twist if foreshadowing is correctly presented, like the steven universe garnet fusion theory. it was so satisfying to catch on to all the little hints and finally have it all come together.
People don't even comment on any of my series. However, if one subscriber was truly invested and correctly guessed the plot, of course you have to divert their suspicions by not giving them a straight answer.
I pull them across the table and whisper you're right but then delete ever saying that so that we will know but they will never have any proof.
In that situation, I would take note of who made the guess, how correct they were, and when it came to that point in the story, make mention that they guessed it right. Foreshadowing in a story exists to not only give it a sense of greater continuity, but also allows the reader to maybe figure out things ahead of time.
It can also be multi-layered, one bit of foreshadowing could hint toward three to five different future events, whether it's directly stating 'X was the most important thing Y could have done/gotten' and having several instances where X is absolutely vital, to the point where not having it would have led to a 'for want of a nail' scenario, or just showing a memory from various perspectives where each are unreliable to an extent, but each gives the reader enough information to piece together what really happened before the big reveal.
28 days later
