There are many, many discussions in previous forums for this question. Below is one of the most thorough I found (thanks to Violet_Z6). In addition to this, if you search "cat deterrent" or "cat problem" you will find many answers.

Cat Deterrents for your Garden:

Keep in mind that each cat is different (like people), what works for one may not necessarily work for another. On the plus side, most cats will keep pesty squirrels, moles and other critters out of your garden. They're great for keeping out moles, rabbits, squirrels, and other critters which can do more damage in your garden than a cat ever will. Birds aren't stupid, they watch for cats and stay away. Sometimes natural law comes into play and the quicker animal wins, it's natural law.

If the cats have owners, talk to them without being confrontational. The cat owner who allows his cat to damage other peoples' property is as guilty as the cat hater who kills the cat for trespassing. Remember, cats will be cats, and it is unfair of us to blame them for being what they are and how nature intended them to participate in this world. After-all, we praise them when they catch mice or rats or other creatures we deem to be 'pests'.

* amonia soaked (corncobs, etc)
* aluminum foil
* bamboo skewers
* black pepper
* blood meal fertilizer
* bramble cuttings
* Carefresh - "recycled" wood pulp
* catnip - donated into your neighbor's yards (so they'll stay in their own yards)
* cedar compost
* chicken wire (metal or plastic)
* cinnamon
* citrus peels
* citrus spray
* cocoa bean shells
* coffee grounds -fresh & unbrewed, not just a light sprinkling (highly recommended by MANY Gardenwebbers!)
* dogs
* electric fence for animals
* essence of orange. essence of lemon, lime (citrus essential oils)
* fresh manure(ditto)
* garlic cloves
* gumballs from the Sweet Gum Tree
* gutter covers
* hardware cloth
* 2 liter bottle full of water (magnifies movement on other side)
* heavy bark mulch
* holly leaves
* keep the area damp, they like dry soil
* lavender
* liquid manure (good for your garden too)
* motion sensor sprinkler
* pennyroyal
* pinecones
* pipe tobacco
* plastic forks
* predator urine
* red wine vinegar
* river rocks over the exposed soil
* rocks, crushed
* rose bush clippings
* rue, an herb (Ruta graveolens) (highly recommended in plant form only)
Scarecrow Motion Activated Sprinkler
(do a froogle.com search or www.safepetproducts.com)
Shake-Away Domestic Cat Repellent Urine Powder
(do a froogle.com search or www.safepetproducts.com)
* short twigs throughout the planted area about 6" apart
* six-inch bamboo skewers (pointy side up)
* Spray on your leaves (not the cat): fill a spray bottle with 1/2 t chili powder, 1/2 t cayenne pepper, 1 t dish soap and water
* squirt gun with water
* talk to your neighbors
* tansy
* thorny berry, lilac, hawthorn, rose clippings
* toothpicks
* upside down vinyl carpet
* vinegar sprayed on areas where they roam
* water bottle on "stream"

NOT RECOMMENDED:
*** chili powder, red crushed pepper, cayenne pepper (NOT recommended), it gets on the cat's paws then they wash themselves and they get it in their eyes, beware cats have literally scratched their eyes out because of this. Even if it's one cat out of 500 infected in this way, that's one too many for me.
*** Don't ever use mothballs or flakes. Those little toxic waste pellets destroy cats' kidney function, could seriously harm people who handle them, and yes, contaminate your own garden soil. Their packaging even warns against using them this way.

Give them their own areas:

(To keep them out of where you don't want them)
(If you don't mind them protecting your garden from other critters)

+ Pick the cat up and bring it to eye level with the plant to see and smell it up close. Usually, once a cat seen and sniffed at the plant, she usually doesn't bother with it later.

+ give them their own plants - i.e., pots of grass for her to chew on and a place in a large planted container on her balcony with some miscanthus grass in it (the cat likes to curl up in that for some reason)

+ if the cats are strictly indoors and attracted to your houseplants, grow catgrass for them. If someone forced you to remain inside one enclosed structure all your life, you might be attracted to the plants too.

+ Barley Grass
+ Any type of "catgrass" from the pet store
+ Carex elata 'Bolwes Golden' but put it in some shade
+ Catmint
Nepeta mussini
cultivars (Simply put, Catmints are Catnips without any culinary or feline use. In any case, they are, however, phenomenal, long flowering, hardy perennials that belong in every fairie or flower garden.)
+ Catnip
Nepeta cataria
(in your own yard) The oils of which also work as a mosquito repellent that works 10 times better than Deet! Catmint is the common name for all varieties of Nepeta. Catnip is the common name for the specific variety of Nepeta called
nepeta cataria
, which is the variety that cats are most attracted to.
+ Cat Thyme (Teucrium marum)
+ Flax
+ Oat Grass
+ Jacob's Ladder
+ Lemon Grass
+ Loose soil and mulch like small bark mulch
+ Mints
+ Purple Fountain Grass so the cat lays in the long leaves all day. Maybe put something in that the cats really like and - you know cats won't winky were they like to hang out.
+ Sandy area
+ Silver vine (Actinidia polygama)
+ Striped Ribbon Grass (can be invasive)
+ Sweet grass
+ Trificum aestivum (type of cat grass)
+ Various Varieties of Cat Mints (Catnips)
+ Wheat Grass
+ Wheat Berries
+ Valerian

As a gardener, grow your indoor cat some catgrass and catnip. They're healthy alternatives for your houseplants and they'll much prefer them.

Change the litter to something they prefer. If you don't clean it out everyday, consider it. Cat's appreciate a clean, comfortable place to go just as much as humans do.

This list compiled from existing GW posts by Violet_Z6, email at violet.z6@gmail.com for comments and suggestions regarding this list.