The ball joint stud has a tapered fit into the a-arm. When you tighten the threads on the stud, it will pull itself into the a-arm, locking it in place. If you are just turning the nut, and nothing is happening (stud is just spinning inside the a-arm), you need to push the a-arm towards the spindle, so it will force the tapered stud to hold. I would imagine the threads on the stud are probably bad, or else it should unthread without any much need to keep the stud locked to a-arm. You could take a dead blow hammer, and knock the a-arm up towards the spindle so it will hold, or you might need to use a pry bar or something to keep pressure against the stud.