This one time, at band camp, Dominic Hargreaves wrote:
Have the framework to enable certain classes of data to be moderated (regexp based on node update for example).
Then you can trivially require moderation for any edit that contains an external URL ref. This would block the vast majority of spam that I've seen on the OpenGuides so far, and would not be an unreasonable inconvience to wiki users.
Or, alternatively, look up the provided URLs in one of the spam databases that tracks such things. Or perhaps start a Wiki-specific one for use on lots of Wiki systems?