Signs attract vandals like cow pats do flies. Cages and fences look awful and do nothing to protect. I have no solution to offer other than these two - which are opposites

1 Anonymity - do not draw attention to rock art by signage "Do not deface this rock art" signs will not work and will be seen as a challenge

2 Teenagers are territorial. They protect their patch. Get them on your side through education. Try to get them excited about the ancient people that carved the rocks etc. Pretty bloody impossible, but it has worked before.