You're going to have to be really patient. A lot of ATC owners have been down the same road with front axles.
First, it's apparently stuck in the brake plate. Kroil (Aerokroil) is your slow, but trustworthy friend. Spray and let soak at least overnight, then maybe spray some more and let soak another day or two.
Heat, cool, repeat. Heat cycling will do more than just heating and beating. Don't beat on the aluminum brake plate!
You may be in for double whammy if the axle is stuck in the bearing spacer. What'll happen if that's the case, is once the brake plate loosens, driving the axle out with remove the bearing with it because the spacer is stuck on the axle. It's not the end of the world and you can cross that bridge later.
You need a brass or at least a brass drift. That's to protect the axle splines. Ideally, a pneumatic with a pointy type bit to center in the divet on the axle, would work best. You can't swing a hard enough to match the cyclic power of a pneumatic. If you have access to a press, that would be your best choice.
The story of three wheels and a man...