After getting comment spam for a while I found my wiki pages were spammed as well: someone added hundreds of links obiously to tune the page rank algorithm of google. There does not seem to be an easy way to deal with this issue. So I needed to disable open wiki editing (which equals not having a wiki at all).
Not only comments and wiki got spammed on several sites – also the referrer pages were full of it.
Dave Winer is trying to deal with the referrer spamming by re-checking the origin. Then Jan Storms suggests a filtering and a blacklist tool. Userland reacted quickly by adding two event hooks for comments and referrers, so that anti-spam tools can be activated in the right moment.