Image by windley via Flickr
When I was building the twitterbot for @utahpolitics, I set up a test account: @uptesting that I don't use for anything. It has 38 followers even though it's just test messages and hasn't had a tweet since early January. The followes are mostly a good list of Twitter spammers or people who follow a lot of people to get a lot of followers.
Setting up a bunch of honeypots on Twitter and then adding anyone who follows them to a blakclist wouldn't be such a bad idea. Someone's probably already built it.