try putting rose trimmings or other ****ly branches in the areas they use the most. You can get sonic devices that obmit a sound they dont like, but i am not sure how well they work, if you catch them in the act a burst on the hose will soon send them scurrying.
I have three cats and i have told my neighbours that if they are being a nuisense to send them away with a flea in their ear, a spray on the hose wont do them any harm.
but they soon get the picture