In a similar vein, I had an idea for a social site where everyone was invited via other people, establishing the chain of invites. Spamming from one of the leaf nodes propagates upwards as bad karma, decreasing exponentially (or by some power, depending on how strict you want to be about anti-spam ruling) with each parent step. If the inviter's karma drops below a certain threshold, he/she can't invite more people. If it drops lower still, the branch dies and everyone on it gets banned.

This is probably not a great user experience (getting banned because someone above you invited a bunch of spammers), but I think it's conceptually interesting to distribute the anti-spam onus to the users of the community instead of the administration in a form other than 'report spam'.