If you have the caliper off and the parking brake released they can be stubborn. Rear rotors tend to seize around the hub and studs. lots of spray penetrant and hitting firmly around the studs with a hammer will usually free it. Noisy but effective. If not, most auto parts stores rent
hub pullers with a 100% refundable deposit upon return. Sometimes parking brake shoes will wear a groove in the rotor making removal very difficult. Before taking any drastic measures, remove the rubber parking brake adjuster plug and retract the shoes all the way.