Cats do alot of damage to neighbors things, on a car it gets scared...scrambles off, scratches on car, spraying and potty all over, not very sanitary to you or if a pregnant women (very dangerous to them)is working in garden, also cats like to attack other cats at windows, now your screen is messed up, so it's not just having them potty in your home with a litter box, they do other damage to property. I advocate keeping cats indoors, I see a thread on here like this one, then next thing I see is someone wants to teach their cat to go "out" to potty so they don't have to mess with the box. Maybe the ones posting these should talk to each other?
I caught my neighbors cat attacking my screen with my GSD trying to warn it off, since it was seperated by glass the cat kept attacking the screen I got it all on tape, you can go to
http://orangedog.forumup.com/index.php?mforum= orangedog
and go to drgnrdr's animals and see the video of it.
For aversives for cats I've got this for you...
http://www.ddfl.org/behavior/cat-avers.pdf
hope you can find one of these that helps.