We were getting a ton of spam on some site blog comments as well. We enabled Recaptcha, instead of the default Subkismet CAPTCHA, and that cut down on the spam, so you might try that. If you're still getting too much spam with Recaptcha, you can try one of the supported hosted comment system providers where they claim to do spam screening for you (Joe's using Disqus for the mojoportal.com blog posts). I think for our site the webmaster also changed the blog settings so comments would only be allowed for a certain amount of time on each post. This way spammers couldn't target older posts with their junk.

Hope that helps,

Jamie