Content Banning Blacklists
To prevent a spammer from saving wiki edits with problematic content, use the variable $wgSpamBlacklist. This should contain a regex for matching on any URLs (or parts of URLS) which you do not want to allow users to link to.
There is also SpamBlacklist_extension(which has a patch for v1.5) extension that makes it easier to manage the regexes and collaborate with other sites.
A friend of mine recently installed Bad Behaviour in his blog and comment- and shoutbox-spam stopped (until now, at least). Although it works well, there is still the problem that it could produce false positives. Might be worth looking at, though.