Any Humane Cat Repellent?

Our neighbor has about two cats and she keeps them outside. They frequently roam our yard and hide under our bushes. Frequent pleading with the owner to keep her cats leashed (city ordinance for outside cats) has gotten us no where. Sometimes the cats poop and kill birds but they seem to like torturing our indoor cat. After she sees them out the window, she works herself into a fuss and later barfs on something which becomes a pain to clean. Recently, the owner even called the police and blamed us for putting a dead bird in her mailbox when it could have been some other neighbor with the same problem. Is there some kind of humane deterrent that's cheap. Preferably something that won't be washed away by frequent rains or look bad in our yard.