Question
I live in the city and cats are using my outdoor potted plant garden as thier litter box how can I stop this activity......

Answer
Hi Claire, Here are a few things to try...
Your local garden center will have a dog and cat repellent in either a liquid or dry form.  Most of these work fairly well if used frequently.
You might try just a liberal dusting of cayenne pepper on the soil.
Covering the soil with lava rock or marbles often works.
I've also used vinyl saucers to cover the pot surface.  For a 8" pot, get a 10" saucer.  Cut a slit in it large enough to go around the trunk of your plant, invert it and slide it around the plant so the soil surface is covered.  Cats are creatures of habit, after a month or so you may be able to remove them.
There are also some sticky products used to deter birds from roosting and insects from crawling up trees.  If you apply this to the soil surface, a little soil will stick to their feet which they don't like.  It won't thrill their owners either. Good Luck.  Jim