Cats are not good receptors of citrus smells. In fact, these smells tend to make them want to leave a specific area. So, a good plan would be to take some left over citrus rinds, and break them up into little pieces to act as feline repellers. Once you have these little pieces, you can go on and scatter them all over your garden. This will make the cats want to avoid that area altogether.
Normally, when cats want to use a specific area to go to the bathroom, they dig a hole in the ground. In order to prevent them from doing this, you can place stones all over your garden. This will make it impossible for the cats to dig in that area, which in turn will drive them away.
Another thing you can do is train the cats. Every time you see a cat trying to defecate in the garden, you can go and spray it with water. This is going to take time, but eventually, the cats will realize that they are going to get sprayed every time they use the garden as a toilet.
You can spread pine cones out all over the garden. Pine cones are very prickly, owing to which cats generally tend to avoid all areas where pine cones are placed.
The last option that you have is to get some chicken wire and place it in the areas where the cats go to the bathroom. This will ensure that the cats can’t get to the ground below, because their claws will keep getting stuck in the chicken wire, every time they try to dig in those areas.