Well I've been battling comment spam a lot here at the SqlXml Blogs. Vinod gets up to 50 comment spams per day. Up until now I've just been using a stored procedure that inserts the offending URL into a banned url table and deletes all the comments associated with that url. New comments are checked against the banned url table and are not added if a match is found.
It was interesting to read today that Scott Mitchell is doing something similar. It seems that everyone is dealing with this problem. I'm sure a good community based solution will emerge soon (maybe something like Cloudmark's Safety Bar which I love).
In the mean time, I've added the Clearscreen HIP-Captcha control to the post comment controls to see if this at least slows them down. So sorry about the extra step in posting comments, if it doesn't work I'll remove it.