Simple patch to UseModWiki or other Wiki engines that automatically blocks wiki spam. See description here: http://www.otug.org/otcgi-bin/wiki.pl?WikiSpamBlocker. This is running here: http://www.otug.org/otcgi-bin/wiki.pl.
Since installing the patch, there has been no WikiSpam.
The idea is to block an edit which adds too many links. Obviously, this might affect some genuine edits, but it has the merit of tackling directly what is unwanted, and (if done well) not requiring constant tinkering as spammers try to get round it. If use of "noindex" doesn't stop spammers, a block of this kind is a good idea.
This is discussed at length on ShotgunSpam.
If they start spamming incrementally rather than in large blocks, then perhaps trick the spammer by allowing the originating IP address to "see" the changes, but nobody else. I know it is probably a lot of programming and storage, but something to think about if bigger guns are later needed.
Another way to cope with this problem is to add a feature into the Wiki server software that generates some sort of image with a distorted (but still readable) codeword for each edit request. The editor must read this codeword and type it into a nearby edit box for his changes to be accepted. See the PayPal registration account page for an example what exactly I have in mind (go to https://www.paypal.com/cgi-bin/webscr?cmd=_registration-run, then click on "Continue" button and look at thebottom of the form. The thing I'm talking about is the "Security Measure" that can be found there: an image, an input box and a blue box explaining what it is about). This is a standard way to prevent robots from submitting data to a system while allowing the humans to do so. If done properly, it will be very effective (especially when combined with the idea of blocking edits adding too many links) as analyzing such images and extracting the text from them is easy for human brain but very hard for a robot.
Also a note to the genuine edits blocking: most people generating interesting content will post it incrementally in small pieces and not as a huge edit. Therefore there are little or no genuine edits being blocked by the WikiSpamBlocker.
This is discussed to some extent in CaptchaTest.