What a horrible problem – you have my sympathies. The first thing I would do it dig out some of the soil where the soiling has taken place. Cats are creatures of habit and once they have fouled an area, the smell entices them back for more. If you can remove the trace it will help to stop them returning. With regards to preventing them from entering your garden, there is a plant called Scaredy Cat (Coleus Canina) which alleges to deter cats from the surrounding area. I have friends who swear by it and others who have not noticed any difference. It may well be worth a try.
In the Kitchen garden we have had some success with ultrasonic devices which emit a sound that pests dislike and stops them from entering the garden. We have used the mouse version and it has cured our mouse problem completely. There is a device specifically aimed at cats which may well be the answer to your prayers. You can find this at https://www.harrodhorticultural.com/mains-adapter-for-mega-sonic-cat-repeller-pid9118.html
I wish you the best of luck. Please let me know if I can be of any further help.