Spambots attacking my forum, need information on building a more secure system.

Welp, rather than making a gigantic explanation, here’s what’s basically happening:

Earlier this week I learned that Kemoy translates into “garbage” in Japanese, so I found a bunch of annoying little bots running roaming around the forums.
Any help on creating a more secure system? I’m trying to avoid activation quizzes as much as possible, but if I have to do so, then I will. And yes, I am using SMF.