Bob and I are both regular posters on an online forum where participants can comment on each other's posts. In the past, it's happened several times that we commented on the other person's post, and our exchanges did not lead to any productive outcome. Eventually, I unilaterally decided to stop commenting on Bob's posts and more generally to avoid interacting with Bob (for example, the forum has an associated chat site with an ignore feature; I ignore Bob).
Bob did not agree to stop interacting, however, and kept leaving comments on my posts which did not abide by the forum's politeness norms. After several complaints, the moderators of the forum told us both to stop interacting. This was fine by me — I just continued what I was doing — but I think Bob wasn't happy with it. He complied at first, but now he's back to commenting more and more, and again with a heavy dose of ad hominem attacks.
It's not the case that Bob is incompetent. Rather, he has his hot buttons and can't or will not refrain from jumping out when duty calls. I don't have a problem with any other user. I don't know if Bob has problems with other users; if he does it hasn't been apparent in the chat.
It's gotten to the point where if I make posts on certain topics, I know that what I would naturally write would trigger a reaction from Bob, so I have to choose between posting anyway and expecting a disparaging comment, or refraining from participating. I'm tired of the disparaging comments. At the same time, other than this one case, this is a friendly, interesting community which I don't want to leave. I don't want to push Bob out, either, but I'm at a loss how to cope with him.
The platform does not offer any way to block one user from commenting on another user's post. The only technical solution is to ban the user altogether (for a duration chosen by the moderators), which would be a pity because Bob does make worthwhile contributions. What can be a human solution? Either involving me, or the moderators, or both.
From my side, “stop interacting with Bob” is what I'm already doing, and it isn't enough; “stop participating” isn't an acceptable solution.
The platform, in case you hadn't guessed, is Stack Exchange. Obviously I'm an involved party, so my presentation cannot be unbiased. I considered asking from a moderator's point of view rather than as one of the users, but I didn't feel I could write an unbiased post that way.