You add the rel="nofollow" to external links in your Wiki or blog. Google (and other cooperating search engines) ignore those links. In time, spammers will get the picture and leave Wikis and blogs alone. It’s a noble effort and I think it’s worthwhile.
For now, I’m not going to touch this, though. I want comments here to be weighed in Google’s search results. Our topic is just so specific and our readership rather small in contrast to so many sites.
When spam becomes a problem, though, external links here will be tagged with rel="nofollow". I’d like to couple it with a technique for approving links.