I know this will sound weird but I have done the following with good results in the past
Use a hair dryer to warm up the steel that the ball joint goes through. Make sure you shield the ball joint rubber etc
Hold a hammer against one side of the spindle, hit the other side of the spindle .
Or
Use a ball joint spreader.
Always leave the castle nut on the end of the threads , it will protect the threads as well as stop things flying apart.