A friends parking lot is regularly full of cat droppings left by numerous felines in the area. As you can imagine, it's quite a nuisance; he regularly needs to wash it to avoid swimming walking in the droppings.
How could he keep the cats out?
Additional info:
There are multiple parking lots in the same underground parking, but droppings are nearly exclusively in his lot. If possible, the solution should avoid just moving the problem to a new victim, but more importantly shouldn't be a nuisance to the other users of the parking.
The cats are presumably strays.
The crimes are committed at night, and he isn't ready to stay up!