It is debatable whether signage could be effective owing to the extreme environmental conditions.... but highlighting the existance of Bronze Age cairns upon ramblers web-sites, in guide books etc can only be of benefit. The mountain tops of Britain are too 'out of sight, out of mind' to ever be policed. However, since your average 'just want to smash fings' vandal isn't exactly likely to climb mountains to get his kicks, I think we need to appeal to the more educated 'vandals' who do. Let's face it, any properly equipped hillsman (the loners or pairs) will, by definition, be prepared for the worst conditions possible at altitude so will have no thought of using - let alone preparing - a storm shelter. Which leaves the larger, inexperienced groups (seeking safety in numbers) as the likely culprits.