Posted: 09/12/2013 at 09:43
Just a note to say that, although they aren't usually audible to adult humans, children can often hear them and find them distressingly piercing.
My partners neighbour had one of these electric noise machines installed to scare off rats. Despite my being 25 years at the time the first time I pulled up and got out of the car the pain it caused my ears made me drop to the floor! It was unbearable to have a window open in his house and I'd be in tears by the time I got to his front door some days! Similar machines are often used in town centres to stop "youth" gathering at night.
I'm afraid I've nothing to add to how to scare cats, it seems to depend on the individual animal as to whether they're threatened by noises / smells (though most will react to a water pistol so perhaps motion activated sensors are the way?)
Are the certain plants that give off smells a cat doesn't enjoy? Google seems to suggest something called "rue" but I don't admit to knowing what it is. Is there a mulch they can't stand?
Perhaps you would be better to guerilla bomb the gardens of those with cats with the likes of catnip / grass to encourage their cats to stay in their own gardens.
What it is perfectly clear (in terms of the law at least) that you can't do however is actively put things down intended to threaten their life (aka no matter what your personal thoughts are cats are not classed as vermin and thus can not be destroyed). Police will act if they are informed that you are putting down poisons such as de-icer for the intent of killing an animal.