Super-soaker and shovel
super-soaker for discouraging cat, easily applied from a good raised vantage point of an upstairs window, no lethal and satisfing....
shovel is for the owners. I return the poop to the owners front gardens or by the door to the car. I find this particularly effective (provided normal channels of diplomacy do not work).
Doesnt stop the cats but it does keep me sane.
Serious suggestion is to put up an over hanging fence (45 degree inward upward facing with some netting, they don't like to climb on it and if they get in they cant get out quickly and are easier to get with the supersoaker, this worked for us. I tried 25cm and that was not long enough 50cm seems to do the trick! For the hard to hearch places pigeon spikes keep cats off fence tops to.
