You have to boil the citrus skins - for about ten minutes to get the active ingredient out - usually add washing up liquid and any other smelly essential oils we have - lavender, burgess ect. Put in in a spray bottle and spray round garden - has to be done frequently and after dew or rainfall. Some cats do not mind the chemical though.
Curry, chili power bought whole sale in big bags and spread round.
Sonic devices - sometimes work but some cats appear to be deaf.
Water sprays that attach to hoses - good but found it leaked water everywhere for us.
For different reasons we put up a 2 meter screen - the main cat can not get over this so has cut off the some one of the access routes into the garden. Main one left is further and means I can get out and hit the cat with water as it scrabbles up the fence. You can get cat deterrents that go on fences - little nails beds.
Some plants often smelly one like herbs can help as does good ground cover.
I know a lot of people are saving it does not matter - but I waste a lot of time checking the garden and the buggers still get in and leave piles - including in DC play house and lawn when the DC come in and out. It means we can not have bark or sand pits. I have had all the young DC at some point get covered in cat shit and then had screaming when I sort them out and get other DC in to stop them getting covered. Plus it stinks in summer. It is not like I can take them to the local parks because those are frequently covered in glass, or burned, or covered in dog shit despite there being signs no dogs and there being ample other space in the parks for dogs.
Sometimes spiky branches works - so I do not see that being that different to kebab sticks though one cat does like to shit on these branches.
None of the cats bury the poo - no idea why and I really resent the money spent especially as the main cat owner the horrible woman next door seems to take such delight in her cats actions.