Here is a Twitter Unfollow tool that is pretty good. You can unfollow up to 100 per 24 hours or pay a very small fee for unlimited. I have used this for a while now and never had any problems.

http://www.justunfollow.com/

When you sign up you will only be allowed to unfollow 50 people a day until you tweet the promo message from them. It will prompt you to do so after you have reached the 50 unfollow limit. I have actually purchased the premium account from them. I use it everyday to get rid of some of the ones that never follow back. It was $4.99 USD for the premium unlimited account which you can only use for one of your Twitter accounts.

I almost didn't add this one because it is not completely free but thought that the free version is still pretty nice with 100 unfollows a day.

Thanks,
Scott