I use a steel hammer and whack the sh*t out of them (copper hammer would be better but I don't have one).
Squirt some CRC or WD40 on the studs.
Make sure handbrake is right off.
Check to see there are no retaining screws.
Firstly I hit the centre near the studs inwards quite hard (the wrong way) to try to break the bond.
Then tap around the outside while exerting outward pressure (the same way that you are tapping) with a lever of some sort (large screw driver or pinch bar)
Work your way around and try to rotate the disc from time to time.
Are you quite sure there are no threaded holes to wind a bolt in? We are talking about the outside face here not the inside.