SMF 2.0 has a number of available anti-spam mods in addition to the built-in CAPTCHA and Questions & Answers. The mods can be used to query 3rd party databases of known spammers. And of course, registrations can be held for administrator approval, and email response required to complete sign-up. CAPTCHA (and maybe Q&A -- I'm not sure) can be required for the first N postings.
The major limitation of Q&A is that there is no language selection -- your set of questions (and their answers) is the same for every language. There's a heated discussion going on about what to do about this. There are also discussions about what other anti-spambot measures could be taken, that won't disenfranchise the visually handicapped (e.g., "what time does this clock face show?"). All forms of CAPTCHA are acknowledged to be easily cracked by spammers.
SMF 2.1 is expected to be out within a few months (but don't hold your breath, either for its appearance or for Lunarforums to get updated).