We can only make it harder and especially more time-consuming for spammers to register on the forums, but stopping it completely won't be possible I'm afraid.
A kind of "symptomatic treatment" would be to give a small limited number of users some extra privileges to delete those forum messages and spammers, since speed of response mostly relies on the time-zone the moderator lives.
It would at least make the forums look clean at most if not all of the time with offensive material very quickly being deleted and spammers removed, so in the meantime we spend time finding better tools or wait for better tools to be available to prevent this.
Say 1 or 2 extra in the US region, 2-3 for Europe and 2-3 for Asia and you have covered pretty much all the timezones effectively.
What do you think?
Cheers,
Martin