you need a thing called a stud remover, it's a 1/2" drive socket with a cut helix inside, you put it on the nut and as you turn to undo it, it cuts into the nut and grips it, so you can remove it.
Irwin make good ones, they call them Bolt Extractors.
http://www.irwin.com/irwin/consumer/...IrwinCat100511