It's neither a completely crazy nor completely new idea. E.g. Metafilter charges (charged? I haven't checked in a while) a $5 one-time sign-up fee that's less for revenue generation and more intended to make it sting a bit if you're banned (which Metafilter is notoriously hesitant to do).

Problem being that Metafilter is unpopular compared to many of the free alternatives. Correlation is not causation, but there is a plausible causation mechanism here.