Well, first spray it down thoroughly with some penetrating lube.
Then if you have a pneumatic pickle fork you can use that to pop it loose.
If you don't, turn the castle nut upside down and thread it on a couple of threads, but not enough so that the ball joint is protruding through it. Secure the bottom of the spindle on a jack stand so that the A-arm is not held by it, then whack the castle nut with a BFH.
Voila! That should do it.
Also, if you have a spare nut, it might be better to use that than risk damaging your castle nut.
Paul.