This is a territory problem - one cat 'marks' the area, the rest 'overwrite' the area with their scent, so you need to break the circle.
Remove all faeces, and wash the whole area down with a very strong solution of bleach and boiling water. Rinse with clear water, repeat, and rinse again. Now scatter the area with mothballs, and place some balls on any nearly arial access the cats may use - fences, low walls and so on.
It will work - but if one of them is persistant, simply repeat the process - they'll soon be off marking elsewhere.