Here are some suggestions that have worked for our farm:
1. Pee around the coop. Enough said.
2. Get a puppy and train him. We have an aussie that does a great job, but there are other breeds that make even better farm dogs.
3. Train the chickens to go back into the barn when the sun goes down. This isn't very hard and most of the time they will do it themselves.
4. Motion sensor and/or electric fence on a timer.
5. Be aware; this is by far the hardest thing.
Our first year we lost 10 chickens. If that's not motivation enough to do something, I don't know what is but I would never hurt another animal for doing what they do naturally.
While I understand that cats are born predators, I can't afford to lose chickens. I wouldn't kill them because they do keep pests down and may belong to someone, but I do want to scare them away from the coop.