Typically surface matter was scraped away to form an image in the exposed, underlying soil, or by arranging stones to form an image. Geoglyphs are the most fragile rock art. As such, it should be treated with respect. Avoid touching Rock Art To do so may have unexpected consequences, such as compromising certain dating techniques. Removing rock art from a site is a very serious crime in many states.
Education is probably the best way to deal with the serious graffiti problem at rock art sites.
Read about etiquette on Indian reservations.