We've got the same problem – cat across the road regards our garden as its own personal litter tray. Anyone who says cats are very clean and bury their crap is lying.
If you catch them in the garden a good squirt from a bike bottle is a sound deterrent. My best hit was when I got one of the feckers when it thought it was successfully hidden inside a tree. Was just heading out on road bike and saw it slip out from under the car (the scene of a recent hosepiping) and up into the tree so grabbed a bottle off the bike and BAZAAAAMMM. Only subatomic particles moved faster than that cat did after a direct hit had been scored on it's face. If I hadn't been picking it's shite up most days over the past 6 months I'd have felt guilty when I realised I'd grabbed the bottle with Torq in it rather than plain water I had meant to get, but all's fair in love and shitting cat deterrence.
Obviously you can't lie in wait for the cat forever and it's hard to get hold of small landmines thanks to the Tart in The Tunnel so we have tried all sorts. Orange peel didn't work. Lemon peel didn't work. The amount of gin we had to drink to match the limes we were peeling meant we didn't care but it didn't stop the cat shitting in the flower beds. Got expensive too. Cayenne pepper didn't work. Tumeric didn't work. Couldn't get lion poo. My mate who brought bear poo back from Canada had his bag searched and it was taken off him at customs. Cat repellant pellets are expensive and only last one rain shower, which isn't much use in Scotland.
So what has worked? Well, ultrasonic cat repeller works as long as cats actually trigger it – good in winter, not so good when there is lots of foliage and cars on the drive so the cat can slip in undetected. We've settled on clear plastic bottles filled with water in the prime shitting spots – that seems to help a bit too – apparently the cat thing doesn't like the reflections while it is squeezing one out (who does tolerate a distraction when performing their ablutions anyway?). It seems to be the best solution and it's almost free so got to be worth a try. Makes your garden look a bit weird but not as weird as having a garden littered with wee piles of cat shite, I'd contend.
Good luck – if it comes to it you've got the perfect excuse to get a dog. A proper pet.