I feel for you, really I do. I have the same problem. Three neighbour cats frequent my garden and all use it as a giant toilet. It is particularly bad at this time of year because I am out in the garden more digging over the beds, etc, and cats do love freshly dug soil to poop on.
I have some cat owner friends, and none of them can understand the problem. They are adamant that cats are lovely clean animals that always burry their poop. They base this on not having any in their gardens - well most animals, given a choice, would not poop in their own gardens, and the cats near my house certainly do not bother to dig holes.
I have resorted to planting very prickly bushes at two corners of the garden they usually enter through (although it will take a year or so for them to bulk out enough to fill the gaps) and barking over the beds that I don't need to dig over yearly. The cats seem to hate the bark - when crossing the garden they now avoid walking over those areas. I think it is because it is bone dry and quite sharp to walk on, so once it it wet with rain that will probably change and they'll start using it like toilet paper.
I can't really use a water based device in my garden because the tiers would require me to site about eight of them to cover all their main routes. I have a shed at the top of the garden with power, but I am loath to use an ultrasonic device (assuming they even work) because the position would cover neihbouring gardens and a primary school, and I can hear the lawsuit already knocking on my door. I won't be sticking plastic/metal cats with marbles for eyes around the garden either after seeing them have no effect at a relatives house (indeed the other cats would happily poop right next to it, almost out of spite).
I have seen sprays that supposedly contain plant oils that repel cats. I have no idea if they work. If the bark and prickly bushes don't sort it for me I might try digging deep holes in the beds and covering them with leaves... or a 12 bore cat repellent. And I am a cat person too!