Most of the people who do crap like this are people stuck in hellish click farms; simple math is easy enough to do and even automate for the bots. I suggest you do something like, I don't know, have a special word or phrase that you want people to input. You could have a sentence prominently displayed on the front page (so that people who are legitimately interested can find it), like "We are X, Y, and Z!" and on the signup page ask "We are X, ___, and Z! What's the missing word?" Bots couldn't bypass something like that, and a human spammer could find out the information, but why would he bother when there are millions of forms and sites that use generic captchas and questions that even a kindergartner could solve?
(I remember someone who ran a political blog focusing on her state. She got a TON of spam messages, and she suspected they were more trying to aggravate her than sell her something, so she decided to require signing up to comment, and a person signing up would also have to answer who the current governor of that state was. It was something that anyone who'd be interested in commenting would know, and spam amounts dropped down to virtually nil.)