I know there are some anti spam plugins out there. I feel it would be a interesting plugin, if the spam comments can be labeled or deleted by the readers. As a result, it will be much more accurate. Using the auto check anti spam and the one 'social way', the blog will be really 100% clean.
Of course, it need sth like
1, if anyone can label and delete the comments, or only registers, or only a group people?
2, a comment will be automatically labeled and deleted if it is required by several readers
3, Giving a new attribute -- give more score for a reader, who labels more spam correctly. Then you may authorize a reader, who has more than certain score, to delete the spam automatically.
4, when i say delete, i mean it will not show up at the front end. You can change it at the admin end.
Hope my idea is good.