Perhaps you can weight the flags of users. Like users who have flagged non-spam content in the past don't count, and people who have flagged lots of spam count more.
Yeah, we've played with that idea a bit. It doesn't help the sparseness problem (actually makes it worse), and if we took action as a direct result, it would give people the power to DoS content they disagree with.
For sites operating at a smaller scale, this could be a good way to surface content for manual review though.
Could you hire people just to review/flag stuff on a part-time basis, ala Mechanical Turk? Or is the problem just not big enough to warrant the time/money investment?
I imagine that while spam is annoying, it probably doesn't impact your bottom line in a big way.
For sites operating at a smaller scale, this could be a good way to surface content for manual review though.