I've noticed that spambots tend to register several accounts in sequence. For example; 12868 (above) was a spambot (in this case the term includes people spamming), so looking on either side of that user, 16866 is sketchy, 16867 is sketchy, 12869 is sketchy, and 12870 was banned.
Another thought for combating spam...though I'm not sure how difficult it would be to implement...restrict account register from unique IP addresses to, say, a certain amount of time between account registrations.