The shaft is a taper with a key. The proper way to remove it is with a wheel puller.... BUT....
Put the nut back on until it is flush with the threads on the shaft. (not tight on the wheel) Have a helper pull straight on the wheel. Then, give the nut a few good taps with a metal hammer.
The nut will protect the shaft, and the vibration from the metal on metal from the hammer, should pop the grip of the tapper.
Don't pull too hard, or hit too hard. It's all mounted into fiberglass.