So use math, the universal language. I had a site request that I try to stop spam on a evaluation/registration page. I added a randomized single-digit addition problem to the form and the spam ceased. A single input field where the answer is always 0 to 18 was all it took.

Or course, if someone took the time to target this specific site, that would be easily thwarted.

It still puzzles me just how this custom-made form got infiltrated by spam to begin with. Do people go around picking such forms and submitting form-specific bot-code code to some vast pool sold to bot operators? Or are bots far more intelligent than I give them credit for?