Live trap is the only way I know of unless you can coup them up at night and make sure there are no holes in your coup for critters to get in. If it is a weasel or a mink a live trap won't do you any good as they'll be able to get through the holes. You can possibly try putting up a hot wire around the coup and turning it on at night. I would do something like 3", 6" and 12" off the ground with all 3 wires hot. Make sure to ground it good and don't forget to turn it off in the morning before letting the chickens loose.
