I checked this again today and it's worse than before!  I don't know who would want to do this. 

It isn't someone who was registered here. The wiki was open to guests (most wikis are that way - even Wikipedia is open to guests), and someone decided to start spamming, most likely with an automated script, although I'm not sure why that one category in particular was getting the bulk of the spam.

I've deleted the spam, banned the IP of one of the users who was doing the majority of the spamming, and temporarily disabled guest editing until I have time to install a spam-prevention thing later (I'm at work now).