Ryan points out a handy tool to help keep even more spammers from making their way to your blogs and posting comments - Bad Behavior.
I love Akismet the spam filter for comments that is built into Wordpress. It's blocked 812 spam comments in a little under 2 months for me. I was content with it, but I grew tiresome that it would even get to my site. Akismet is great for blocking spams that are already to your blog, but there has to be something out there that would block the spammers from coming to your site in the first place.
Luckily there is something like that, it's called Bad Behavior.
Luckily, it's also made for Wordpress, so you can download and install directly with no fuss. He comments that he's already seeing results with even more caught than Akismet was before.