I've heard that too, and I don't know if it will work.
But if it's easy to set out a live trap, it's probably worth trying.
If it would be an expensive nuisance to get a live trap, probably not worth it.
Can you maybe get a live trap and set it next to the coop? If the fox is pacing around the coop and goes into the live trap to investigate it, it could get caught, and dispatching a fox in a live trap would be much easier than trying to get one that is free to run away.
If the fox doesn't get...