Get a couple of your own - they will defend their own garden against other cats.

We tried allsorts but in the end our solution was "Thomas The Tank" a beautiful rescued greyhound. We have had him a year now and have not seen a cat in all that time. Works a treat.

Don't know if this really works, but I came across the plant called 'Coleus canina' - also called Scaredy Cat plant - in a T&M pamphlet. Apparently it's got a foul smell that drives them away but only smells to humans when touched. Wear a gas mask while planting it?

Sorry Bulbaholic - having cats doesn't seem to work. We have 3 cats and they're always entertaining the neighbours cats in our garden. On Saturday morning, I found the nighbours Bengal sitting IN our pond having a drink with eyes fixed on the Koi below, the other neighbours grey moggy comes strutting across the garden then through the conservatory and into the kitchen to pinch our cats' food. Then a new neighbour with a cat has just moved in - he/she has also discovered our garden. Often our 3 can be found in the garden with all of their new-found friends.
All I'd say is, please don't use anything dangerous - the pain some pets go through, and with vet bills being so expensive....you'll never stop them.