i rounded the head on one of those bolts so it was effectively stuck. if you're not afraid of using a grinder, the solution is fairly easy and quick.

all i did was grind the head off the bolt (the other i was able to unbolt as usual), and then the caliper bracket came free. finally, i was able to unscrew the remnants with a wrench.

of course you'll need a new bolt.

good luck.