Imma assume nhiu does know the author can see - some comments such as “tag properly! Some suggestions...” seem aimed at the authors, so. They’re definitely a dick but I don’t think there’s much anyone can do?
There is one thing you can do about it! Make a positive bookmark for that same story (if you’ve read and liked it). Tell your friends to do the same. 20 bookmarks and that ugly review will vanish from the first page, and that means SO MUCH to authors, as does seeing positive bookmarks.
You can’t stop this user from leaving their bookmarks. AO3 is pretty neutral, by design, and that means bad things as well as good things. There’s a reason you can’t directly message users, and it’s to stop flame wars and abuse.
But you can take action one story at a time. Knock that ugly bookmark back a few pages. Tell the author that YOU like the story and share it with your friends. Leave a positive bookmark of your own. And out in the community (tumblr, twitter, wherever you hang out), be sure to preach that bookmarks should be either positive or private.
All these things help, and none of them are about dogpiling an individual, which is good, because no one digs in and gets defensive and no wars start.
