I've recently often witnessed the quite sad phenomenon of new (and often clueless) users trying to make themselves known by adding useless answers to already-answered questions, or to old unanswered (and often unanswerable) questions.

This is becominig increasingly common, and increasingly disturbing; the most disturbing effect is the fact that any question that receives a new answer gets bumped to homepage, cluttering it even more than it already is.

How can this problem be mitigated?