actually captcha's do not stop bots at all. in my personal experience, the only thing that works is logic questions such as:

If the sky is blue, does that make an orange green?

the answer would be no, and the bot is confused because there are two questions inside of one question.  another good example would be


"I apologize for the length of this question, but how secure is to secure? please type three spaces in the box followed by the very first word on this page. "


these are very easy to setup with SMF forum.

sources: running forums for 3 years now.

total bot signups with captchas: 11,968
total since removing captcha and adding logic questions: 0