Every time we talk wheel spacers it seems like 1/2 the forum believes that they fly off all by themselves!
If you are 100% sure they are steel… maybe try a magnet on them… then a bigger torch and for longer will help, as will soaking for several days in penetrating oil.
Another method, absolutely tried at your own risk, could be to loosen the retaining nuts/bolts by 1/2 of a turn or so and then remount the wheels. A few hard turns or even a donut under load should pop them free…
I would lean toward penetrating oil soak for a few days, heat one side to try and get differential growth, and slam them sideways with at least a 2+ lb hammer, dead blow if available. Hammer until you win.
If they are outboard of the rotors, hammering back toward the axle center on the flat rotor surface can also lever them off. Drums? I don’t know…