This forum already has a Captcha. Captchas are old technology and there are many bots which are designed to answer them, I remember reading a few months back there are Bots which answer captchas correctly more often then humans.
Simple questions can reduce numbers but there are Bots which would correctly answer very simples ones, a slightly more unique question would block 99% of them I suspect.
FIshy Bishy, just checked your scramble puzzle and that would not stop some of the newer Bots.
Then there are human spammers like these two guys which are difficult to block.