Sometimes, here on SE sites, I have seen a behavior that I consider abusive. I'm sure that many other users here also saw that. I will use an anecdote to present it:
Some users (let's call them Alex, Beatrice, Carol and Denis) are debating something in a comment thread of a question or answer. Let's say that it is something on meta about defining some policy and all of its hairy intricate problems, but the debate so far is healthy and productive with a lot of useful and helpful information being produced.
Then Eris steps in and posts a comment presenting a controversial and unorthodox point-of-view in an impositive and aggressive manner.
Denis and Carol engage the debate with Eris' and the debate becomes heated up. Alex and Beatrice try to continues the once healthy debate by initially trying to ignoring Eris, all of that happening in the same comment thread. However, Eris' insistence eventually derails the debate into chaos. Beatrice gives up debating and goes away. Carol bites the bait and does some personal attacks and/or mocking on Eris. The comment thread becomes a huge mess.
Eris, unable to convince (read: impose) other people of her opinion, then deletes her own comments. Without Eris' comments many parts of the thread do not make any sense anymore. To make things worse, Eris flags everything from everyone else as being offensive, rude, obsolete, off-topic or whatever other lame excuse fits in. To improve the flag count, Eris also either uses a sockpuppet account to also cast flags or calls in a friend to help her with that.
Fred and George come in, see the attacks that Carol made against Eris and flag them as well. Denis, Alex and now Fred even try to resume the debate, but they are now struggling to make any sense of it. However, they don't get too far into that, because soon, a moderator steps in, nukes all the comments and suspends Carol.
Eris achieve success. If nobody is going to agree with her, then everybody should shut up! The then healthy, productive and helpful debate was derailed and then nuked to be lost forever! Also, a once productive and helpful user ends up being suspended and everybody else is frustrated. Eris is laughing.
I won't post links to actual real examples of this because I don't want to expose anyone and most of what would be examples of this are deleted anyway.
I know that comments are intended to not last (an idea that I personally strongly disagree with, but I will not enter in this merit). But the reality is that many healthy and productive debates happen in comments and yet don't fit in answers, being very vulnerable to the above kind of escalation.
A chat room could be an alternative, but chat has its own problems like (a) ordinary users can't convert an already going-on long comment thread into a chat except under very special occasions that very rarely apply, (b) flagging in chats does not work well, especially if a user is only presenting a controversial position that everyone disagrees with instead of being blatantly offensive, (c) it requires users to open up a new window/tab instead of seeing all within the corresponding question or answer and this UI/UX barrier ends up being a cognitive obstacle that inhibits many people of participating.
Now, the questions:
What can I do to stop users like Eris that are trying to derail the debate without being blatantly rude or offensive to anyone?
If the debate was already derailed, what can I do to save what was useful and resume the healthy and productive debate later?