Someone spraypainted some boulders near my house that had ancient petroglyphs on them, how can i remove?

the spray paint grafiti without harming the petroglyphs? the petroglyphs are the type where they were "tapped" into the rock,the rocks have a thin coating of black oxidised rock and under it is lighter ,thats how they made the petroglyphs, so harsh scrubbing with a wire brush would ruin them and i think chemicals might react badly as well, any environmentally safe ways to get the paint off? these petroglyphs are over 900 years old it would be a shame to just lose them. is it maybe just better to leave the spraypaint there? it will eventually (in a hundred years or so)wear off on its own but that might be better in the long run for the petroglyphs. what do i do?