If it is tightened down all the way and still spins there are really only two options=replacement of the arm or tack weld the flats of the ball joint to the arm. If you do weld it do not give it sustained heat or it will destroy the new ball joint-just a quick tack at each of the flats,waiting a bit between each flat for it to cool down. The old owner of a front end shop once told me that and said if you put the grease fitting in it and give it a squirt of grease that will help prevent damage to the ball joint. There isn't much in the way of "threads" in the arm so once they get buggered it's easy for it to strip and sometimes you can dent in the arm from the outside to give it more resistance but it may end up popping out of the arm after driving(with car weight on it)-I had that happen to my Charger.
