I'm a big believer in notices. I know people say they can spoil the atmosphere or they're unwelcome bossyness but it's quite possible to place them on the route to the monument or in some fashion whereby they're definitely seen but not intrusive. And not only will they hopefully stop the 99% they'll also make the 1% think twice maybe, knowing that everyone watching knows they're doing wrong. I also quite like the idea of English Heritage or National Trust snipers. Imagine what a satisfying job that would be...