Find a socket slightly smaller than what is left of the bolt head. Pound the socket on the bolt head, it will result in a very close fit and should not strip off. (If the socket is too small it may split.) If the bolt threads are the culprit then you may shear off the bolt head. You'll get a lot more torque on it this way than with a flat blade screwdriver.

Many of the tips here are great for dealing with the difficult case of a bolt threaded into a part, but the fasteners that hold the rotor to the hub are actually a nut and bolt (much easier to deal with).

Just cut the head off the bolt (dremel) or drill into it with progressively larger bits until the head pops off.

The remaining portion will fall out the other side.

cut a slot (or cross) across the bolt head with a dremel and use an impact flathead or phillips screw driver and hammer.

Francisco