TweeMaid is a paid service that cleans out people that you are following but that are not following you back. Or you can ask them to have them unfollow everyone. It does not matter if you have one follower or a million, it is a set price of $15.95.
You can add exceptions for people that you want to continue to follow despite the fact they are not following you.
It is a nice housekeeping tool to order from time if you don't have the time to go through your list yourself.