We're pretty darn serious about the quality of the posts at craigslist, and really want each one reviewed and removed if it's problematic. The culture of trust we have results in very few problem posts, given our size, but they happen.
However, we're getting around 1.5 million posts per month, and we also believe in turning over control of the site, as much as possible, to the community.
When faced with that, we started evolving mechanisms which turned over posting quality control to the community. First, you could flag ads for our review, that worked pretty well for a while, but then the volume proved to be too much for us. Now, if enough people flag a post for possible removal, it gets automatically returned to the poster, with limited explanation.
We haven't done this yet for our forums, but will figure something out.
This system isn't perfect; no democratic system is, but it's withstanding most attempts at abuse.
We've been pretty satisfied with it. Today, at breakfast, someone told me it's actually a real innovation on the Internet. I'm not sure if the innovation is flagging, or the decision to turn over control to the community. In any case… more to come!