Couple of things...
I upgraded both of my forums to version 2.0 of SMF. There were some features in 2.0 that allows questions to be asked on registration that prevent spammers from signing up in the first place. As bots become more sophisticated at solving Captcha, other measures were needed.
For instance, you can ask a question on registration such as:
"What game is this software primarily used for?" and if they answer anything but "poker", it will not allow the registration to proceed. That feature has been great on the forum sites I run.
The plug-in module I use is "Stop Spammer" version 2.3.9
It allows for easy checking of email and IP addresses to a database of know spammers. It has helped me prevent a couple of accounts from joining the forums.
Hope that information helps.
Cheers,
Matt (Bixby)