Wiki spam is rubbish (usually an advert), which is added to wikis by robots. Its very similar to e-mail spam, and just as annoying.
I've only had a few pages affected so far, but if I get more, I'll have to put in measures to prevent them (such as forcing people to register before they can edit pages).
I have changed wiki engines. I used to use JSPWiki, and now I use my own wiki engine called Pinkwino, and I haven't had any wiki psam since then. This isn't because my wiki engin eis better (although it is!), it is just less well known, so wiki spam bots don't know how to find, and spam my wiki (yet).