They sell graffiti remover and Home Depot. I'm not sure about your rock but I used that and a wire brush to get it off brick. Maybe a nylon brush would be better.
Do not try to remove this yourself. You will almost certainly damage the rock further.
DO NOT USE A WIRE BRUSH OR ANY CORROSIVES ON THE ROCK!
Contact the BLM in your area about the graffiti and your desire to remove it.
Its people like yourself that keep our crags clean and pristine. If everyone cared a little more then maybe this topic would never even have to be discussed.