They ought to come up with a spray product that disintegrates the spray paint right on the spot with no fuss involved !
It already exists; however, you can't use it on walking surface.
Help your customers solve graffiti problems with Sherwin-Williams Anti-Graffiti Coating. This unique siloxane product creates a non-stick surface that repels graffiti from paint, paint spray cans and permanent markers and doesn't require solvent cleaning or abrasives for removal.
The conventional versions are not intended, however, for a surface you would walk on.
Though there are some products out there that claim to protect granite pavers without affecting slip resistance. I don't have any real experience w/them.