There is a product called cat off (or something like that) I think it's citrus based. Pretty sure bunnings will have something like that or similar. Maybe spray a border around your property? Another idea is maybe call into a vets and ask ideas? sometimes they have advice. our local vet was really helpful in suggestions to keep the neighboyr's cats away from our chooks. We have the most grass out of our block and any one time we will have 5 cats in our yard. Today there were 2 dogs too lol. I only own one cat.

HTH