Craigslist phone verification system: Craigslist has implemented perhaps the absolute killer app in terms of anti-spam solutions:
[…] Craigs*list has implemented a phone verification system in certain categories to reduce SPAM content. When you attempt to make a post, it asks for you to verify yourself by typing in a viable phone number. It automatically calls it and repeats a verification code which you must then enter for your post to go live. If your post gets flagged, the phone number you verified with will be blacklisted.
So while email address are cheap and disposable, phone numbers are not, and every spam post is bound to blacklist a phone number.
The link goes to a forum newsgroup thread where a bunch of spammers talk about it and conclude that they’re more-or-less screwed. There’s a lot of talk about getting cheap VOIP numbers, but the closest workable idea they found was to go to public places and use pay phones.
This presumably has the added benefit of binding a set of spam postings to the same phone number. So, if one of them gets flagged as spam, you could find everything else verified from that number and pull them down at the same time.