Occasionally users who are new to the site post questions without quite grasping that this isn't a traditional threaded message board. I've seen numerous occasions where these users try to respond to answers by posting another answer (in fact three times on this question).
It is understandable, but adds clutter. As a possible solution to this I propose a way to flag these types of answers, perhaps as "should be a comment".
To be nicer to the new users, it probably shouldn't penalize or scold them, but maybe it could either (a) move the answer into a comment on the main question; or (b) put some kind of a notification on their profile that tells them it has been removed and asks them nicely to repost it as a comment on one of the answers or the original question.