We recently eliminated the spam from our Contact Us form with a very simple implementation. We added an input that was labeled "URL:" in the HTML form and made it invisible to the real users. Then, in the form processor, we check to see if it has a value and act accordingly.

The spambots take the bait all the time; they put in a URL to some spammy site. Our script sees that and throws away the comment (actually, we recycle the bits because we're trying to be a greener eco-friendly sort of company). For a while, we'd still store the offending comment in a database table for review but would refuse to email the results anywhere. That's how we know it worked.

With this simple method we went from around 30+ spam "Contact Us" messages a day to ZERO.

Good luck with whatever you choose!