It's not just the GearHead wiki as there's SPAM on this forum, too. For that matter it's not just a problem for GearHead. There has been an EXPLOSION of spambots across the entire Internet. And Wiki's in particular are being targeted by those would would profit from SPAM ad revenue.
This is becoming a big enough issue that Google has taken notice. Google has a team working on the problem and investigating how SPAM infiltrates wiki's. I happen to know about this because I participate on the Dwarf Fortress wiki and read their SPAM reports.
From what I understand, it's becoming difficult for many administrators to keep on top of the SPAM. And they've tried all sorts of tricks and techniques to stop it. The CAPTCHA technique of identifying a sequence of letters and numbers as well as trick questions - like "what is the color of grass" - only go so far. (Most SPAM comes from 3rd world countries where the pay is pocket change.)
These days, I believe it can take rather drastic measures to thwart them. In particular, I'd recommend changing the code so that permission to post a link is disabled for new users until they make several posts or contributions.
Yes, it's often useful to have links on a wiki. But there are ways of having your cake and eating it too. For example, the DF Wiki has exemptions in the wiki code so that even a new user can share a DF Wiki link on their DF Wiki and they can post a link to their forum, but all other external links are blocked for new users. (They also prevent new users from modifying certain key pages, such as the main page.)
As for the SPAM on forums, if it gets bad enough I'd recommend having new users type in a short message explaining why they want to join and require someone to manually approve new members. For some online communities this was the only method they found that could thwart them.
