Open "Following" page on Twitter and scroll down to have all following accounts showed in the browser.
Next run the following JavaScript code in your development console:
$('.button-text.unfollow-text').trigger('click');
http://www.quora.com/How-can-you-unfollow-everyone-on-Twitter-at-once
Next run the following JavaScript code in your development console:
$('.button-text.unfollow-text').trigger('click');
http://www.quora.com/How-can-you-unfollow-everyone-on-Twitter-at-once