I don't have any good ideas for option a), and I dislike reddit-style downvoting for option b) since it has the same problem of people upvoting bad stuff, just in reverse.
One idea which could work for b) is a digg-like 'bury' option (i.e. a 'this is crap' button) - but only in tandem with a high karma minimum for being allowed to use it, and some statistical cleverness for deciding when a post should be buried. But I'm terribly enthused with this idea either.
Really, I don't think we should be necessarily banning content, but rather encouraging the community at large to post worthwhile material for itself.