I have 4 cats (5 until recently) and understand how horrid it must be to find cat poo in your garden ... I hate it in mine, but obviously can't complain (and have in fact made a dedicated, unlanted, soft earth patch for them to use, which I then re-bury very deeply on a regular basis).
However, I have made a point of asking my immediate neighbours to tell me if it gets to be a problem in their gardens - I have offered to come round and clean up, and have also said I have no problem with them turning the hose on them to deter them. So .... if you know whose cats they are, it may be worth having a word. They could also encourage their own cats to use their own garden, as I have done, by providing a cat friendly space.
As others have suggested, citrus peel is a reasonably good deterrent, but you do need to replace this quite often as the citrus oil dries out and evaporates. Alternatively, some people reckon moth balls put them off. There are also physical barriers you can put down ...... eg. lots of twiggy stuff between plants, so they can't get on the bare ground in between. I appreciate this doesn't stop lawn problems, but IMO, it is very unusual for a cat to do it on any sort of hard ground as they are normally fastidious about burying. It might be hedgehog poo you're seeing .... they do quite big poos for a small animal, or even foxes (which really stinks).
You can get a plant called "scaredy cat" from the garden centre which is supposed to repel cats. Another idea is lion poo which you can get in pellets (Google it if the garden centre doesn't have it). I know it sounds like you're replacing one lot of poo with another, but it's actually sterilised before it's sold so it's hygenic, though the smell that remains of the lion frightens little cats. It's a fertiliser so it will improve your garden at the same time.