Those bolts can be tough.
It is easy to round out the hex heads resulting in lack of "bite" of the hex wrench.
Heat as John suggests.
It is possible to take an appropriate sized socket driven torx driver and hammer it into a rounded out hex fastener; basically turning a hex fastener into a torx fastener. That has worked for me a few times.
Another trick is to pick up some valve lapping compoud from the local auto parts store and fill the hex head of the fastener with the stuff. Then tap your hex wrench into the fastener, the lapping compound provides additional "bite".
Lastly, when all else has failed, you drill out the heads of the fasteners, remove the brake rotor. Then heat the cut off fasteners and try to purchase with a ViceGrip, or drill a hole in the fastener and use an extractor (eazy-out). This last option is not preferred, and is what to do when you have mucked things up so bad you have no other option.
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