Cut a screw driver slot on the threaded bottom of the grip screw stud with a Dremmie tool, you may even able to salvage the stud. If all else fails ... put a "death grip" on the grip screw stud with "ye olde VICEGRIPS" OR clamp it in a vice. Last option woud be drilling a hole in grip screw to remove the head.
Everyone with a 1911 has done this once. Then they learn to RED Loctite the stud in place (it can be removed with some heat later if required) and justa SMIDGE of BLUE loctite on the grip screws to discourage them from coming loose wthout torquing them down so visciously.