Question for a friend?(a kitty poo poo problem)

Sorry if this is the wrong place, its not really a behavorial problem or anything, it is just that a friend of mine has neighbors cats that poo poo right in front of her porch!!

Is there anything she can use(of course SAFE!) to keep them away from there??

I don't know of anything offhand. She might try putting cat attract litter in some other place that she doesn't mind if they potty. I have no idea what else might do it.