Use a master lock universal trailer coupler lock
How to use a master lock universal trailer coupler lock The Master Lock universal trailer coupler lock is a simple device really, the key locks and unlocks it, when you unlock it the slide comes out where the ball would be in the coupler. When you want to lock back on to the coupler you push it back as far as you can and turn the key and it won't go anywhere. This video shows you how it works with few different couplers
