Blacklisting those entire sites would seem to be a real disservice to the legitimate content that is uploaded, but then again, if the site administrators can't keep their content in order, why are they getting crawled by the G so often anyway?
The real deal is this, in many cases: the spam that's created on those sites during certain times is foreseeable and follows a particular pattern -- it's created based around news or events, such as sports, movies, sex tape outings, etc. -- so it would seem easy to get rid of the spam before it hits the search engines, right? One option is just to expect an influx during certain time periods and increase moderation during that time. Another option is to prevent new users from publishing content until they've submitted moderator-approved non-spam content first.
But that would mean those sites don't get the impressions... and impressions-->ads-->$$$... so where's their incentive?