It could be very annoying receiving hundreds of registrations daily on your forum and most of them are spam (bot registered member) but with the help of this method, you are 98% safe from spammers. Q&A is what I've been using and it is working for me up till now. Why not try it instead of using some untrusted mods. To do this in your SMF Forum goto: Administration Center » Security and Moderation » Anti-Spam and set up your question. Remember: make the question simple in order to encourage members.