So, it seems twitter is blocking mass unfollow tools, but is there any still available? doing it one by one is tedious =/. Socialoomph can't do it either. edit: nevermind www.buzzom.com does the job.
You have to be super careful with unfollowing. It's the quickest thing that will get you banned. Even if you're churning (unfollowing people who don't follow you) there is a limit you want to stay in, more than 100 a day or so and you're asking for trouble.
I used buzzom all the time, it just unfollow 50 at a time versus how it was before when you can unfollow thousands. It's safe as long as you don't unfollow 1,000 in one day.