If you've been hammering on the threaded part of the tapered shaft (even on the end) you've probably bulged the thread to the point the nut won't install properly. You might have to run a die onto the ball joint thread as well as replacing the nut. Next time hammer on the boss of the spindle where the taper fits. Hit it HARD with a big hammer there, but never hammer on the nut or the tapered stud. By distorting the taper slightly, it will pop the stud out of the female taper. I've been removing ball joints and tie rod ends this way for over 40 years, and virtually never damage a thread.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!