Spambots are smart. Apparently there's a lot of money in plastering comment threads with ads for counterfeit watches, penis pumps, etc. The CAPTCHA system we use on DNN to test for humans is pretty good and catches the vast majority of spambots. But it's not perfect, as we saw just today when someone vomited Ugg boots all over our site.
I'm thinking of switching over to a new, more interactive, sort of test, like one of the following: