[mcav]

Groups does have a 1-5 star rating system; I've noticed spam often gets one-star votes in groups I frequent. But that doesn't really address the problem; they ought to have a "report spam" featuer as well.

[davidw]

So the spammers just start reporting a bunch of good emails as spam in order to confuse the moderation system.

[jacoblyles]

This actually happens in webmail quite a bit as well. Spammers open up a bunch of accounts, spam themselves, then mark their messages as "not spam". But the major vendors have figured this out.