Orange juice / Lemon juice are utterly hated by cats. They can't stand the smell. Mine won't even go near me after I've eaten an orange, until I've thoroughly washed my hands!
Try shredding some orange / lemon peel and placing it around the gravel, with the highest concentrations in the region around where they usually come in (and / or where they usually drop their moggy-logs if they have a favourite spot). You really just need to get them looking for less unpleasant places to crap - once they've found a new spot you can stop putting orange peel down.
Also: Put it in a bin / Mustard / Nuke it's ovaries from orbit etc.