The blacklisted wordslist really worked for me in fighting email spam in a 500 user environment.
Every time you get spammed, blacklist the ip address and some of the obvious words.
This is a hell of a job to begin with, but once you get started on this, and work on this once every day or so (even though you will only this for half an hour), you will see a dramatic drop, well i saw a huge drop, i filtered about 60% of the spam that would otherwise pass.
Perhaps you can find some other wiki's that have the same problem. Like wikipedia?
I also read something about a trusted environment. Begin by trusting a selected view, and have those trust others easily. This will generate an environment where it is still easy to "wiki", and have less impact of spammers.