They generally don't like citrus. You can slice some lemons or other citrus fruits and scatter them in the area you want the cats to stay out of. It worked for me when I was trying to keep our dogs from digging up a young herb bed.
Rosemary, lavender, garlic sprays, clove and peppermint oils will also deter cats.
Placing plastic forks, tines up in the garden will also deter them.
The best idea I have used is pine cones. They do not like the feel on their feet nd it makes a great natural cover on the ground. Not to mention it looks great!
If you have a cat shelter or rescue organization close by, they might have a a trap, spay, and release program, which will cut down on the population. Any of the younger ferals that can be socialized might be put up for adoption.
They make motion sensor activated air canisters that blow air at them. After a few times they won't come back. Learned that from the show My Cat From Hell with Jackson Galaxy.