There are many web applications that can help you to manage your twitter account, show you who is not following you and help you to unfollow or flush them. Some of these services you have to pay for but some of them are free.
I have been using Tweepi to manage my Twitter account. With Tweepi you can easily find followers who do not follow you or who follow you but you do not follow them back or followers who do not tweet much or you can just clean up your twitter account if you have too many followers. The basic service is free and that is all you need to unfollow people who do not follow you with just few clicks.
Tweepi is also easy to use. You sign up with your Twitter account.
Click on ‘Manage Twitter’ on top nav menu.
Then you will get the list of all the users who do not follow you.
Choose which user you want to unfollow and then click 'unfollow'.
Your Twitter account will be updated accordingly.