A few months ago, comment spam finally started being a problem on wonko.com. Up until that point I had been doing nothing more than setting a cookie to weed out automated spammers that didn't support cookies. Amazingly, that actually worked for quite a while, but the spammers eventually wised up and started supporting cookies.
My solution, since I'm lazy, was to implement the dumbest Captcha in the world: a static, unchanging image of the word "pants". The word never changes. It's always "pants". Nevertheless, I haven't had a problem with comment spam since implementing it. Go figure.