reCAPTCHA
Unfortunately do you know math has become popular enough that a few spammers are getting through. I don’t really understand why they would do this, since even if you get through, I vet first time posters before the post goes through. In any case, I just discovered reCAPTCHA, and am trying that out. It should be pretty straightforward, but just in case people are having trouble: there are two words, one should be clear and one fuzzy. You only need to get the clear one correct, don’t worry about the fuzzy one.
This also helps digitize old books which I find very exciting.