To hold the Hub I am thinking what would happen if you put the Brake Rotor and the wheel back on and lowered the Wheel down on a block high enough so that there was room to get under the Car. Apply the Parking Brake as layback40 said.
I believe all that would keep the Hub from rotating while you try to turn the Retainer Ring.
Also Penetrating Oil before and inbetween heating can be helpful. However, becareful as it can be a Fire hazard.

I suppose an old Brake Rotor could be turned into a Hub Holding tool if a Handle was attached to it.

__________________
84 300D, 82 Volvo 244Gl Diesel