Had the same problem, the local tom cat kept bothering my kitties and spraying till we got Leo our Rottweiler. He persuaded the bully to stay out of our yard in short order!
You might have to keep your cat in the house, I know that is very difficult to do. Maybe over time they will learn to coexist, took my two three years to tolerate each other. Other wise the vet bills might get pretty expensive. Are the cats fixed by the way? That is a absolute must! We have enough homeless animals and they make much, much, better behaved and happy pets(don't mean to lecture).
Check your local pet store, they have some pretty good odor eliminators now. Nothing smells worse.

We were staying in a village in Italy for Christmas this year and the cats would come up on the small porch and use it for spraying and making messes, because the apartment had been empty much of the year.
We noticed that other residents put clear plastic bottles (2 liters) filled with water out side their doors. Many years ago I had heard of this as a deterrent for dogs that sprayed on lawns here in California, so we tried it and it worked wonders.
Evidently the animals think they see movement behind the bottles (the wall, steps, etc.) and think it is another animal and so avoid the area. Laa.

Cats kept pooing in my polytunnel. Every pot had a thick coatint 1 inch think mould on cat sh**

I put battery operated pressure sensitive mats with 100db alarms all round the polytunnel. First day it was very noisy and irritating, but I have never heard them go off since that and now have a cat free polytunnel.

Cat Deterrents for your Garden:

Keep in mind that each cat is different (like people), a deterrent that 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. 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
* 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)
* 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. She noted that once her cat has 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 mussinicultivars (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

This list compiled by Violet_Z6, email at violet_z6@yahoo.com for comments and suggestions regarding this list.

I was at the grocery store and found a clove of what they call elephant garlic. It' just a jumbo sized bulb of garlic. I took a couple of the bigger pieces and cut the into slices and wiped them all over my door and front steps and the neighbor hood cat hasn't come back.