I encountered an interesting capability on a forum that uses another "engine" besides SMF. During the registration process, you are shown a series of pictures, and required to put check marks next to the pictures of animals. I think it might be effective to decrease bots.
Is this something that SMF can do?
Here's the link to the forum:
http://www.stykz.net/forums/
which uses phpBB.
I've attached a screen shot of the bottom of the registration agreement where the "confirmation challenge" is.
i dont know of any mod for smf that is like that