I'm thinking it's just one person that's signing up repeatedly. There's a captcha phrase on registration that should block all bots. Blocking by IP is easy to get around too, if they really wanted to.

My guess is that they are trying to boost their Google rank, or search engine visibly, by posting more links to their sites. This ups the page rank in google, the more links there are to a particular page.

Hopefully, they will figure out it's a waste of his time to keep posting spam, just to have it deleted.