I am all for enabling a user to filter their own experience, however. So if you are ignoring someone, it would make sense that their comments would drop closer to the bottom in your view of a post. As for general moderation, if and when we get into that, I'd like it to be as dependent upon user input as possible. Ignoring does give a pretty good metric for that, at least to raise a flag that there might be an issue.
I'd ignore them, so their comment would sort to the bottom, and if others ignored them, the same would happen. My guess is that they would run out of steam eventually. If we don't take the good with the bad, then we are left to determine which is which. If it became real problem, I'd probably consider some other approaches, but I don't think civility can be enforced very well.
You're right about civility, it can't be enforced.