I'm pretty sure I installed Akismet for you. If not, it's a simple install, then set up your account with them. (Add the plugin and it will walk you through the process.)

It's a Bayesian anti-spam service for your comments. I get 100k+ visitors per month, and I get maybe one or two spam comments get through per month, with about a dozen per day caught.

Don't try to solve this one on your own. Akismet works.