UNfortunately this is far too common, and these sorts of resources are often either as yet undiscovered, or if known, often spread around vast expanses of land in fairly remote areas. There's simply no way to police/guard these sites. We've got a similar problem here in the southwest US with poachers cutting entire sections of rock wall out to steal petroglyphs. Often on public land, often on private (reservation) land.
These are rare and finite resources, that even when on private property have a value to society that's much greater than any $ amount. Perhaps anyone caught stealing these sorts of things should be committed to spending the rest of their lives doing conservation/protective work for these sorts of sites.