'My friend's' work occasionally ends up in the public sphere. One newspaper reported on a study he did and completely misinterpreted what his team concluded. Anyway, he woke up one morning to death threats and general abuse. It really fudged with his head and ended up relapsing on benzos. Trolls really aren't worth the effort: just shut him out of your life. If it's causing you so much anxiety that you're asking for help/advice, then as I think some others have suggested, you could either block him or if that's not possible just agree secretly with your friend network on the un-moderated site to move to a more private communication platform. I'm new to this site, as is my friend (we both joined to seek advice on addiction), but from what I've seen so far it seems pretty friendly (and well moderated). Whatever you decide to do, try not to let the 'see you next Tuesday' cause you distress. At least you and my friend have learned that you're not the only ones struggling with trolls!