I have gotten broken bolts out using a very small chisel or very sharp center punch. I don't punch the center of the bolt and then drill it, I gently punch the outer edge and set the sharpest part of the tool in the dip and gently tap it to the left holding the punch as low as possible. A tap only spins the bolt a hair, but if you can get it to move just be patient and slowly work it out a couple of threads so you can get your fingers or pliers on it. Like was said above, if the bolt moved freely in the threads going in then it should spin out fairly easily. If you do damage the block threads a bit just get a tap and carefully chase the hole to clean things up.