2 further possibilities:
First, could you smother all the possible entry points of the intruder cat - fences, walls, ground-level gaps, etc - to your property with pepper, especially cayenne pepper? Of course, with the way the weather is, this could get washed off very quickly, but if a cat gets cayenne pepper on its paws, it'll hate it and maybe not come near again.
Second - and this is probably not a great solution, but... any chance of getting a small dog, such as a terrier, to add to your retinue? It will not please the cats at first, but they could probably learn to get along together. Arrange with the cat's owner to give you a small piece of cloth that the cat has slept on, and let the dog get the scent in its head. Then give the dog its own basket, and station it right next to the cat flap. The dog, if it's reasonably intelligent, will learn to recognise the cats that belong there, and let them pass unchallenged, but be very aggressive to the one that doesn't. If that cat knows that there's a hostile dog just inside the cat flap, it'll think carefully before attempting entry.