If a "grippable" amount of the stud is out of the head, use a vice grip. If not, you drill a hole and use an "easy out". An easy out reverse threaded so that as you're turning it in a direction that would remove a bolt or screw, it's turning into the hole that you've drilled (grabbing the piece (stud) tighter the more you loosen it). You can purchase them at Autozone. Just make sure you get one small enough to fit into the hole you've drilled.