Few things to try on top of the suggestion of strong smells.
I have a cat, and at least 3 other cats come into our garden. Sometimes these things work, sometimes they don't.
1. First thing is to have a sacrificial area. You are basically giving them a litter tray outside. It will be a bed or area, with very loose sandy soil, which cats prefer pooing in. Let whatever pioneer plant species are able to tolerate it grow in it. Occasionally use a shovel to remove the poo. If you have the space you can have a dedicated animal poo compost bin which can be spread on ornamentals or tree crops, otherwise, it just goes in the bin.
2. Grow densely. Cats like to poo in bare soil, plant densely and they will be less likely to use it.
3. There will be times, where plants are sitll growing and the soil is less covered, so when this is happening, stick a whole bunch of sticks and twigs upright in the beds you don't want them pooing in. Cats like to be comfortable and a fairly lazy and will tend to avoid using this as their toilet.
