I think its completely unfair for cat owners to let their cats wonder about the city, causing car accidents, potentially getting killed by other animals, and intrude on other peoples property.

Another alternative would be to tie your kitty on a line whenever it goes outside if you REALLY need your cat to go outdoors.
When I had my cats, and they were in good health, they both went outside for at least 6 hours a day, when they felt like it. I think it's important for cats to get out and about and explore surroundings and my cats benefitted from it. A cat on a leash is a bizarre idea to me, I have heard of it before but I just don't get it. My cats had a much more fun life rather than being stuck indoors their whole life, but I'm not going down that line of indoor and outdoor cats. Of course, I can understand if a person lives on a particularly busy road or if there are dangerous stray dogs about, and of course they MUST be fixed, but I have heard excuses of 'I'm worried Kitty will get scared' or 'Kitty might forget where the house is'. Cats are intelligent and sometimes I feel people don't give them enough credit for their ability to be independant.

Back on subject, the supersoaker is a good idea. Cats despise water (usually) and will remember that for a long time! The cats will hold a BIG grudge against you, but they won't come back in a hurry!
Also, you can buy certain scents to spray which deter cats, and also you can buy special cat detectors. They are a small box which is sensitive to movement. When a cat walks past, it emits an ultrasonic sound which deters the cat. The sound is inaudible to people and dogs.

I hope you find a solution soon.