Few dogs seem to like skunk musk, may be an effective deterrent...or just take the dog back home and turn back on its collar.
THis is tough, you certainly dont want to punish the dog. I don't know how else you can make things clear to the owner. There's no easy solution if they continue to turn the collar off.
We had nearly the same problem....Tell your neighbor you will have to shoot the dog if it doesn't stop.
The base unit that gets turned off is inside the house. If it was on the collar, I would have done that long ago.
You could just set up a small electric fence for livestock. One or two times hitting that thing and he will be gone
I sprayed the neighbor's dogs with fox urine I bought at a gardening store. The stuff reeks to high heaven. Have not seen that neighbor's dogs back this way since.