MWpro - 11:43 pm on Jul 8, 2009 (gmt 0)

I would be careful not to breach any contract with them. How long is the current contract for?

Other than that, I recommend reading "The 4 Hour Work Week." He has a chapter in there on dropping clients who annoy you too much or take up too much of your time as you are making less money per hour with these clients.