+1, or is it +2, on taking the fork out and dissolving the stem with a lye solution. It will remove any paint on the steerer but it will leave the steel steer tube in perfect condition and won't cause any bulges, scratches, etc on the tube that are possible with brute force methods.

Do be very careful with the lye, it is obviously quite nasty stuff and you don't want it anywhere that pets, children, yourself, etc can knock it over or get into it.