Unless your good actors are willing to conform to a published set of (nerfed) behaviors that don't have the possibility of being bad, or are willing to register and be vetted by you individually, you can't help but be overwhelmed by false positives. It's the same reason why the most intricate, pervasive, and technologically flexible surveillance system in history can't find a terrorist.
edit: I think the entire endeavor is doomed. The bubble associating your current searches with your past searches, and attempts to eliminate spam and eHow through algorithms have just resulted in eliminating most sites from the searchable internet. You don't realize how bad its gotten from all of the search engines until you spend an hour on something like millonshort (which looks like it's down now.)
You can actually identify opportunities by finding valuable queries that are being squatted on by the content mills. Lots of nice things in stuff like home improvement, insurance, and finance.