If they have an owner(s) inform them of this problem and request that the use the kitty box inside the house instead of your property which means keeping them indoors. I hope you get their cooperation with this answer of mine because nothing else (sonic beams, water spritzers, irritants, repellent, etc ...though someone will tell you it works for them) will keep cats away once they desire to use your property as their toilet. Remember it's not the cats fault, it's the owners fault. If the cats are feral there will be no potential for resolution at all.

A good solution would be to pack the border with plants so there is no soil to be seen, planted out properly with carefull pruning of the plants so it does not look over congested can work well, for clients of mine over the years I have adopted this method with good results, on large areas ground cover roses work well, on smaller areas you can create a tapestry of heathers or other ground hugging plants etc.

I use the metal shelves from an old mini greenhouse and the plants grow through them. It works really well.

Julien and Cammomile have both given you good advice. It you leave soil/ground clear then cats will poop! There is no way to actually stop a cat from doing this unfortunately. Given that we are cat owners we know! our rule is keep the ground well covered!

Until you have the soil all covered with plants try covering it with eg holly leaves or other prickly stuff.

Cats don't like citrus. Scattering citrus peels into the flower bed will repel them, orange peels, used lemons, limes, grapefruits, etc.

Rose prunings placed across the soil,and a few stood at an angle,at the front of the border,works well too..It sounds a bit cruel for them to get pricked by the thorns,but they only do it the once..lesson learned..Also ,I have used teabags in the past with either Jeyes fluid,or a well known Eucalyptus oil,used as a nasal decongestant..sprinkled on them...they don't like either..Replace periodically,when the smell has disappeared...

I have tried citrus, Bathgate, but found it didn't work for me.

My (nearly) daughter in law swears by a product called Get Off my garden. After having a smelly garden she tried it and no cats have left packages there again. Might be worth a try. If you can, have a look at it on Amazon and read the reviews. Almost all praise it highly. I believe it's also available at Pets at Home.

Anything sprayed with citronella oil or plant some lemon grass is also very effective at repelling cats and other critters.

You could try 'Silent Roar' its pelleted Lion poo which is supposed to make them think theres a very big cat in the territory and they leave...in theory. I dont know how well it works, but at a tenner a box, its worth a try. And as others say...dont leave any areas of exposed soil. They take that as an invitation! :)

My cat hated Six x fertiliser and never went near it.

What a great tip Merlinbaby....I will remember that!

Of course, you could get a Jack Russel Terrier! lol!

They need to go somewhere. I put a bag of fresh compost in a corner of the garden and they use this. I can remove their deposits easily and they leave the rest of my garden clean

Makes sense Janpled

My cat always buries it in a discreet location...behind the Yew bushes or Pyracantha. You never know when it happens. I have onions, peppers and marigolds planted in the vegetable bed and that seems to repel everything.

Our aged mummy Siamese now uses a litter tray and daughter prefers spots in the shrubbery so we don't see anything either Bathgate. That said a couple have just moved into the neighbourhood and have a cat, that isn't being let out yet so we'll need to see what happens when it is.