Image by Clover_1 via FlickrYou can now remove credits from sites by assigning a negative number of credits to it. The credits will be removed from your site and added to your unassigned balance. The same can be done for banners and text ads.
In fact, you can rearrange the credits assigned to your sites by subtracting from some and adding to others in one go - all subtractions are performed first.
This makes it easier to maintain your sites and keep the right ones in rotation.