On a NEW balljoint, the joint is tight & stiff, so it doesn't spin real easy. Also the threads on the stud & castle-nut haven't had time to corrode. So you can spin the nut on without worrying about anything.

After a few years the ball gets easier to spin in it's socket - it's supposed to do this, but NOT supposed to get rattly-loose. Sometimes a joint is right at the point where it's easy to spin but it's got no sideways rattling looseness.

Now if you re-use a good ball joint, it might spin. All you have to do is press it's tapered stud hard into the tapered hole, so friction keeps it from spinning. Some cars you can get a small jack under the knuckle or under the control arm, depending on which way the BJ is oriented. Other times (upper?) you have to get creative with crowbars or C-clamps or something. If you're lucky the stud has a screwdriver slot or a socket for an allen wrench so you can hold it. Once it begins to get tight, then it sticks & you can continue to tighten it with a torque wrench.