Cats are territorial, so they are marking your garden as their area. If you remove faeces, they will simply mark again. So ...
First of all remove all traces of faeces and scrub the area twice with boling strongly disinfected water. This removes the scent traces.
Then scatter the area with moth balls, also around your garden perimiters, remembering tops of walls, fences etc.
That should send out a 'no go' message to the current incumbants, and discourge any newbies from taking over.
The moth balls last a few weeks, rain permitting, and simply replenish when needed. Once the problem stops, you can just put the odd moth ball out, but keep them up on gravel areas, cats think they are a giant litter tray, so these are favourite marking reas.
Good luck.