If the joint spins when the castle nut goes on, I just push the joint in harder until it's held enough to get the castle nut back on.........no big deal. If the joint is loose enough to turn/rotate it by hand, replace the joint.

You can get some replacement ball joint boots at Pep Boys.
They come in bubble packs with a red background that says "Help".
Next time, contact me. I have the tool that pops joints out of the control arm, (or tie rod ends) without destroying the rubber boot.

I also pop them loose when I do a free ball joint test for people.
I show them the difference between a good joint and a bad one,
and they can SEE if they need replacing or not......I DON'T do the
"shake 'em and sell 'em" trick.

Only when the b-joint is welded in (usually to the top a-arm, if at all)
only when welded, do you need to replace the arm. If it's the spindle end,
most times it is pressed in or has a snap ring. If you need a hydraulic press, get back with me. I have one.-Bob