There's a number of anti-spam tricks floating around. I will hopefully be compiling a list of the most interesting ones very soon for my own reference. In the meantime, this one seems like a simple enough hit that can get rid of quite a bit:
(As posted on the Daily Irrelevant) …If your IP address is on a list of known open proxies (mostly used by email spammers, but recently by comment-spammers as well) you will not be able to post comments.
[via Matt]