Ideas for getting rid of urine in loose rocks?

Cats have been urinating on loose, porous rocks in front of a neighbor's ground-level window. I have read that vinegar and enzyme products can be used outside, but would that work on rocks and dirt? How much would need to be applied and how often? Any other ideas?

I'd save money and try first to water it down very well - like a long drenching rainfall. I can't see enzyme products really working on dirt. You'd be going thru gallons of the stuff.