Broken Bolt

While trying to replace the water pump, one of the bolts into the engine block sheared off. We were working on the bolt: 1) one of the bolts was rusted tight; 2) an impact gun rounded off the nut head; 3) we then used a bolt-grip socket head, which was when the bolt sheared off at the nut. We are unable to pry the old pump off, which should leave the stump of the bolt sticking out, it only shifts slightly. We don't want to scratch up the engine block by sticking a pry bar between the pump and the block. What do you suggest we do to get the old pump off?