Spray penetrating oil on it (PB Blaster). And, either put the blade back on and block it with a 2x4 so it doesn't turn or use a pneumatic impact wrench to reverse the nut. You can also put a steel pipe over the threaded end of the spindle and smack it a couple times with a hammer (so you jar the nut, but don't bung up the threads) and/or heat the nut with a propane torch (so it expands and fits more loosely. The last couple suggestions just make the nut easier to remove, you still have to block it or use the impact wrench so the spindle doesn't spin.