Don't spraypaint over it. you make it worse.

Sometimes you can get appropriate solvents to remove it. but blasting it works.

Been their and done it with a friends of trail crew. - sandblaster on ATV/horseback. maks you *really* hate graffiti.

The only people I know with more of a hate for it are the guys in the local caving society who go clean up one of the accessible caves every year - They haul stuff down to blast it off!..