First, I ask them to elaborate.
Communication, communication, communication...
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If they can put their experience into words in a more constructive way, I thank them for that.
Not everyone knows how to give a good critique, and even good critique can hurt. But it's helpful to know what people think.
But we also have to keep in mind that not everyone is going to like what we do, regardless of how well they may or may not be expressing themselves. Tastes differ.
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If they just keep going in the same way, being hateful etc, I'll tell them that I hope things take a turn for the better for them soon. Which I genuinely mean. It's not a snide remark from my side. I believe that people who seek attention this way, or feel the need to use the comment section as an outlet for negativity, are struggling with something - one way or another.
I'm not saying this justifies their behaviour, or serves as an excuse.
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At the end of the day, you should never feel like you have to deal with this behaviour if you can't / don't want to. Check in with yourself, and put your own mental health first. <3
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The only approach I don't feel leads anywhere, is answering negativity with negativity.
Then it is better to block/ignore and move on.