Rubber hammers don't free rotors, and usually neither do claw hammers. You need a small metal sledge (no less than 4lb) and some aim. You're trying to vibrate the rust & brake dust out of the mating surfaces, which is why it has to be (heavy) metal-on-metal action.

If you smack the back of the rotor, you risk damaging the wheel bearing, stressing the tie rods, etc... and actually warping the rotor so that the center part clamps on the hub even harder.

Drench everything with your favorite penetrating oil (pb blaster, deep creep, etc...), let it sit for a few hours, than bang between the fastener holes in a star pattern like you're trying to hit a home run.

Sometimes it takes a few whacks, sometimes it takes 2 people an hour or more.

__________________
A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.