Step 1

Fill plastic bottles with water and place them strategically around the garden with the caps off, perhaps some near the gate and a few near the fence--wherever you suspect the cats walk. When the cat brushes against them, they will fall over and may get the cat wet. Since cats dislike water, they will associate the garden with getting wet.

Step 2

Spread a repellent wherever the cats make a mess. Cats usually inspect the site which they plan to use as a litter box. An unpleasant and irritating smell will deter them from using a spot in your yard. You might need to "refresh" the area every few days to keep the cats away. Some other repellents that you can try:

crushed red pepper
pine cones
commercial cat repellent
ground up orange peels
white vinegar

Step 3

Plant something thick where the cats usually dig, preferably something low-maintainence and drought-resistant. While the plants are growing, stick plastic forks firmly in the ground. Place enough of them close together so that the cat finds it difficult to turn around or dig in the area. This can be a temporary measure until the plants become established.

Step 4

Cover the surface. Lay down a sheer barrier to keep the cats from digging, such as chicken wire, window screen, or stones.

Step 5

Invest in a motion sensor sprinkler. This is a more expensive solution and can take some getting used to (in terms of remembering to turn it off before you or anyone in your household roam the yard).

Step 6

Set up an animal trap and bring it to your local animal shelter.

Tips

Search in google, for example, 'How to keep cats off my lawn' or 'How to repel animals'
Talk with the cat's owner-they may consider making some changes as to where fluffy will be able to get out of their garden.