Jimmy Wales on battling wiki spam
Sure, I think it’s pretty simple to solve problems like that. One of the first tricks I would try is to parse the wiki text that someone inputs to see if it contains an external link. If so, then only in those cases, require an answer to a captcha.
Second step, keep editing wide open for everyone, but restrict the ability to post external links to people who are trusted by that community. Make it really easy for trusted users to extend the zone of trust, because you want to encourage participation.
Basically what I think works in a wikis is to trust people to do the right thing, and trust them as much as you can possibly stand it, until it hurts your head and makes you scared for what they’re going to break. Because that is what works.
People are not fundamentally bad. It only takes the smallest of correctives to take care of that tiny minority that wants to disrupt the community.
I’m glad to say that so far the css-discuss wiki spam problem has been effectively tackled by a hard working group of dedicated spam fighters. Helping out is as easy as signing up for the recent changes RSS feed.