This one time, at band camp, Christopher Schmidt wrote:
MoinMoin already has one; see http://moinmoin.wikiwkiweb.de/AntiSpamGlobalSolution for information on how it works (or how to turn it on, but it's only got a plugin for MoinMoin listed there.)
That doesn't seem to be working. Google cache: http://tinyurl.com/66gek
This is a nice blacklist of regexes for Wiki spammers: http://blacklist.chongqed.org/
Of course, it's the wrong approach. It's not scalable in the same manner as scalability problems with RSS. Having a delay between refreshes to combat scalability issues means you could get spammed by an already-known spammer.
Should be some database that is query/response based where your Wiki submits all the URLs and gets a boolean reply. RBL-style approach.
Some very basic, quick-and-dirty regexes could possibly solve much of the problem. Look for the drugs, gambling games and watch brands they're pushing. Instead of giving a "this is spam, rejected" message, give a cryptic technical-sounding error like "server timeout" or "internal server error".