Richard, depending on your blog software (package or custom written), you should take a peek at the Akismet libraries out there: http://akismet.com/development/ It's free to use and provides external spam decision making based on thousands of authors and other factors. It's really quite impressive... unfortunately there is nothing quite as good for trackback spam which my MSR blog has been getting hit with ~4000 pings per day.
Akismet support was just sitting there, so i grabbed a wordpress API key and turned it on. now to come up with something for the phpbb2 installation, that's already being hit since i straightened out its issues. sigh.