A very sharp , small centre punch to the outside edge of the bolt . Angle it away from the softer material of the caliper and give it a good clout anti clockwise . will take a couple of attempts to ping in a good purchase , but guarantee will shock it into movement once the punch point digs in . Then find a hex key that actually fits , using the wrong size Hex bit , or a cheap set is the worst culprit for this ! Hex bolts are far superior to headed bolts . just need quality keys to get a perfect fit ! good luck .