Oh how I hate comment spam. I have tried everything and it just keeps coming. Now I’ve found a site with the best way to kill it for your WordPress 1.5.x install: CJD Spam Nuke. You install it, activate it, go to the comments moderation page, then nuke every single last lame posting. It happened so fast I had to check it twice to make sure it worked.
If you have comment spam, go get this plugin and kill it dead. I did.