Sometimes there is a little round clip on one of the wheel studs that holds the rotor on. If you don't have this then it is corrosion that is holding the rotor to the hub. If you spray penetrating oil around the studs and let it soak. A plastic hammer or brass hammer will usually work to smack it off. You did remove the caliper?
When you do finally do get it off remember to put a thin smear of anti-seize between the surfaces. I do the same thing on my alloy wheels, those will corrode to the rotors also. No fun getting a flat and the wheel won't come off.
