The real problem is that most "idiots with cameras" (not, I hasten to add, meaning the OP here) just don't know how to turn the s*dding flash off - you only have to look at the Olympics where endless flashes are going off all round every stadium - when the cameras would actually take better pictures with the puny, ineffective flash turned off.
So if I was trying to perserve a historic site I'd just ban all photography, because any attempt to just say 'no flash' is doomed to fail because of the rank ineptitude of the great unwashed majority of camera (mis)users.