Add a "recycling" symbol underneath each comment thread that exceeds two or three comments.
If all participants of the comment thread click the flag, the conversation is removed.
Participants remain in control of their conversations, but get a real opportunity to clean up noise - this was very difficult until now (people had to @ each other to delete their comments now, something that was so difficult to do that it was almost never done in practice)
Participants can @username each other to initiate thread recycling
Older conversations are unlikely to be cleaned up this way
Only whole comment threads can be cleaned up. Suggesting selective, "range-oriented" cleanups would be more desirable, but would add a lot of complication to the UI
Just thought I'd throw this out there as an alternative possibility.