From another thread:
Mandrew wrote:I've added a CAPTCHA that asks questions. Hopefully that will help. My version of MediaWiki came with ComfirmEdit, the extension that allows for CAPTCHA, but it can also be easily downloaded and added manually.
I have no idea what goes into the admin end of a wiki, so I can't say how easy that'd be for someone who's unused to messing with that kind of thing, but doing so ought to eliminate the bulk of the spam.
And if CAPTCHA is a foreign term... it's an acronym of... something. The gist is that it's supposed to be a simple test to tell a human from a non-human. You know in some places when you register, you'll see an image of some distorted number or letter combination, and you're asked to type it into a text box? That's a CAPTCHA. People can make out and enter the corresponding string, while the distortion prevents automated software from doing so.
Some CAPTCHAs require some kind of intelligent response instead, by prompting the user to answer some simple question (like, "What state is the Statue of Liberty located in?").