Yeah better to bring the Akismet plugin into the fold as @eviltrout is currently doing (after tagger) rather than building out a lot of redundant text detection stuff.
I can see a use case for blocking arbitrary text with a custom message, this would fix your problem with the inserted default text links, too, @cpradio. Kind of a dangerous feature in some ways but if we say you can’t block text unless it is 6+ chars in length, less so.
In general spammers, as dumb as many of them are, are still a bit smarter than using the same URL over and over. We banned spam URLs for years at Stack Exchange and the repeat hit rate was tiny. Once the URLs are burned they tend to switch really fast.