Hose it down with lots of WD-40 every 10 minutes or so for a few hours, then with a plumbing torch, heat the HUB, but NOT the shaft. Hopefully, the heat will expand it and allow you to tap it off lightly from behind against a block of wood.
Another option; there should be HOLES around the hub inside of the cooling fins. Weave a CABLE through them and around some loose HEAVY OBJECT, like a sledge hammer. The cable should be long enough that you can get a reasonable swing AWAY from the rotor before the cable stops it.
Maybe combine all of the above if one alone isn't enough. If the object you're swinging is heavy enough, it should definitely pull that rotor off.
most likely, the reason its stuck is because the key is rusted. It also could be stuck in the shaft. If it is, do NOT try grabbing it with vicegrips. Take a hacksaw and cut into the end of it, parallel to the surface of the shaft, and pry against that cut with something like a crowbar.