You can use ManageFlitter for fast and easy unfollowing on Twitter, but is a catch: although it lists all your followers in pages of 100 with checkboxes, but without  a "Select All" option.

Twitter put their foot down on "Mass Unfollowing" in April, 2010. All twitter supplemental  applications either had to remove "Bulk Unfollow" options or the "Select All" option to continue to operate within Twitter’s updated Terms of Service.

The "select all" function  is just a JavaScript call and happens in your browser, so all one has to do is find a plugin/add-on for the browser to allow the "select all" function. More >