Had this happen to one of my fronts when i was changing the rotors. Keep soaking them in PB Blaster every 30 minutes for a couple hours (I fell asleep cause it was late after like 4-5 hours), then i woke up and did it 4-5 more times, then was able to bang them off with a hammer. Put some muscle into it! Note: If you bang your rotors off with a hammer, they WILL need to be replaced.