I recently replaced my tub spout but I can't get it to sit flush with the wall. When I try to tighten the spout onto the plastic nipple, the plastic nipple just spins on the copper pipe. The plastic nipple is screwed onto a brass fitting that was already attached to the copper pipe.

How do I get the brass fitting to stop spinning on the copper pipe? I can't even untighten the plastic nipple now, because the two of them just spin together. Help!

The threads on the plastic nipple appear to be stripped. Replace the plastic nipple with a brass one of the same size and See if that doesn't work better.
Good luck, Tom