Apologies if you already know about this.
For my forums I use Stop Forum Spam -
http://www.stopforumspam.com/It is kinda like a community resource. You precheck new registrations against their list, and if you get a spammer that's not on their list, you log the details so others will gain from your experience.
They have scripts that plug into most forums to more or less automate this process (Resources menu on their site, Mods and Plugins). I don't know how the SMF plugin works, but I can see one exists.
Free to use, and I have been using them for years, and it turns administering new registrations for a site into a fairly straightforward thing. I reckon it catches about 95% of my potential spammers, which means I have to eyeball fewer registrations.