Use one bolt from each of your radiator brackets to put in the two threaded holes on your rotor (if it has them) and just tighen them a little at a time alternating between the two!
if that doesn't work you'll just have to tap the back of the rotor with a hammer a bit in that rear location where the caliper was. Just rotate the hub a bit every few strikes and it should work free. I only recomend this because I assume you dont care about your old rotors.