Except for trying to scare them away with bad odors, i think ultrasonic sound devices are a good alternative.
Most of these devices have a motion detector so they turn on when an animal is detected and then make this ultrasonic sound which scares away the cat. Most humans can't hear it except for maybe young people because they have better hearing.
It will also scare away other type of animals like dogs, foxes, birds, mouses, ...
Put the ultrasonic sound generator where you find the poop of cats.