I bought a solar powered sonic thingy last autumn to use in my front garden. I have had about 99% success with it, in that I have only had to deal with fouling twice in the year. I should explain that the front garden is now laid to slate chippings so it is quite difficult to clear anything up.
The small back garden has now got artificial grass, mainly because I can longer cut the grass so there is nowhere a cat can go.
Next door's cat is the only one that comes in and I frighten it by clapping my hands and shouting at it. I don't know whether it really works, but it always clears off
now when it sees me.
Don't you think it is unneighbourly to have a cat these days who is not trained to use the cat litter ?