I recently read a blog entry on "The Coding Horror" (http://www.codinghorror.com/blog/archives/000712.html) in which the blogger (Jeff Atwood) explained that he had added an extremely low-tech captcha to his submission form-- the same jpg every time. He finds that for his purposes, this works-- it stops 99.9% of his comment spam in his blog, simply because there is a captcha.
Granted, it may not be the most sophisticated method, but why not try this before you shell out for a high-powered solution?