Cats use their faeces to mark territory, so they will return to the same place unless you discourage them.
Remove all faecal matter and scrub the area thorougly with hot water and strongly scented disinefectant. Let the area dry, then repeat the process. This removes the scent which encourages the cat to re-mark the area at intervals.
Next, get some moth balls and sprinkle them liberally over the 'toilet' area, and also around your garen boundary, including any ariel entrences - fences, walls, garage roof and so on. Maintain the mothball barrier for a couple of months, and your regular 'visitor' will have moved on.
If the problem recurs, then you need to start again, but usually neighbourhood cats check out all likely areas, and if they find yours is 'barred' to them, they won;t bother coming back.