So, as a basic way of blocking the spammers, how about just rejecting anything with "http://" in the Title of a node?
Philip Greenspun had a nice little social engineering tip about abusive posters to web forums. When they'd locked out a particular IP address or login, instead of getting a "nyerr nyerr you've been banned" message, they'd allow the user to read, but when trying to post they'd get a technical-sounding error like "resource busy" or "database not available". The abuser would just think the site was shit and give up, instead of working out they'd been banned and finding ways around it.
Not sure if this will work with Wiki spammers... I'll be dropping the entire netblock of this wiki spammer at the firewall tonight.