One problem, however, is spam. Virtually all forums are plagued by spam posts and user registrations, creating hours of tedious work for the administrators.

I just found an SMF mod that implements Akismet for the SMF system: SMF-Akismet.

If only I’d found it a year ago, when I really needed it!

I will take this opportunity to point you in the direction of the Akismet Development page. If you head over there, you can find libraries for PHP and other languages, as well as plugins for other CMS and forum scripts (like phpBB or Joomla). Akismet can be used nearly anywhere that comment spam is a problem.