Unfortunately, putting an aluminum spacer onto a steel hub is a recipe for galvanic corrosion. This is more than likely why it's stuck. I had to cut a spacer off when this happened on my E36
Try spray the WD-40 or PB Blaster onto the seam between the hub and the spacer. (usually this is where they get stuck, not on the aluminum rotor face)
One trick you can try is to carefully drill into the spacer and then thread the hole, and put a bolt into it. Sometimes heat will break it loose as well (someone suggested a torch). Aluminum expands much quicker than steel, so heat the spacer and try to knock it off, or try a few heating and cooling cycles to see if it will break free.
Be careful with a chisel as it's bad to damage the aluminum rotor hat.