I am aware that proposal to add flag "Comment is an answer" has been discussed and declined. Perhaps, it is time to review that policy, and add following mechanics: when comment is deleted due to being flagged "Comment is an answer", it is replaced with:
Answers posted in comments are removed
For example, we can replace only first comment deleted as answer, and all following answer-comments deleted without a trace.
I would like to have a feature that would communicate to people that comments are only designed for clarification of the question, not for extended discussions or answers. Right now, the only feedback people get is when moderators move conversation into chat (which can be done only once per question AFAIK). Another feedback is people commenting "please don't post answers in comments". People pay attention to their own comments, so maybe that can be used for education.