We also have this problem. I think our neighbourhood cats pick and choose where and when they will poo, as it’s not an everyday occurence. I used to think cats buried their poo, but read that dominant cats do not, and a more submissive cat will bury theirs, so the dominant ones don’t smell them. It’s a sort of marker for the others in the area.
I tried Lions Roar, which worked, then ran out.
Get Off my Garden, Cat and Dog repellent,is another good one, and smells quite nice. It is a sort of gel crystals.
I tried chilli, cayenne and curry powder, as a mix, don’t know if it kept the cats away, but made me sneeze.
At the moment, I have covered any bare earth with lavender clippings, smells lovely, but not sure if it will work. It’s the bare earth that they seem to like. Little devils. I can’t get too cross, as we have a most beautiful little cat who visits us, and is so friendly.