Are you trying to dismount the tire while it's on the car? Or are you trying to remove the wheel/tire, so that you can put the spare on?
If it's the latter, the wheel is seized to the brake disc. Since you can't get it off from beating on it from behind, call a tow truck, hopefully you're not on the emergency jack. Hopefully you have AAA or roadside assistance on your car insurance, then they can try to remove the seized wheel from the brake rotor, in a safer manner.