I'm sorry to hear you are having to contend with this.

There used to be a humane ultrasonic motion sensor activated device, some are solar powered, that the humane society recommended. I had to get this to keep them out because my one dog years ago, though friendly with my cats, would kill others that came in the yard. It really helped. I would imagine they are still available. Some of them even had lights on them as an additional deterrent for other wildlife.

Some have used cat pheromones to keep others out. It marks the yard for territory so they don't want to come in. Sometimes though a really dominant cat though will mark over it though so that still makes it their place. It is less expensive though than the ultrasonic but could be worth trying.