My cat was scared of my brother - maybe you could use him.
I heard that the sonic scarers are the best. But they are strange animals and pretty unpredicatable - a super soaker would do the trick, but you would have to lie in wait for a long time to catch the brutes.
I like cats and dogs, dogs are easy to understand - cats do whatever they like when they like.
A nearby neighbour has an ultrasonic one, that has a motion sensor in it so it only goes off when something is nearby. It does seem to discourage cats from hanging around his garden.
Thank you for your answers. I also think that a shot gun is the best but not in the city. My gran's solution was to saturate the deposit with hot pepper and vinegar, make temporary friends with the cat then rub its nose in it, spank it and drop then watch a cat doing a streak of lightning. My daughter who has tried three supersonic scarers is too friendly with her neighbours to do that but the occaional spreading of chilli pepper is discouraging but washes away and they are back. She has the only back garden on her side of the street.