I recently found out that at my base on Red Rocket.

Nick was wearing my power armour, Obviously without my permission.

This can be really anoying since I cant make him take it off.

It forces me to go back into the wasteland to find another power armour.

Nope just talk to them and tell them to get out of the PA

Edit: to make sure no other settler steals a PA remove the fusion core when you leave the PA in the base