Awesome-- I'm suffering for a bad case of referrer spam too. A quick way of fixing it for me has been to use phpmyadmin and run a couple of commands to delete the records of spammers that often pop up for me.
DELETE FROM `pixelpost_visitors` WHERE `referer` like '%spammer.com%' LIMIT 10000;
Where spammer.com is the domain of the spammer. I ususally grab a ton of domains of hte spammers and then run it all at once.
Just thought it might help out some people who have a mysql database thats eating up space with referrer spam.
I can't wait to see this problem tackled as well as how you've accomplished tackling it.