So how to catch them? Place a CCTV at every ancient site in these Isles. Er, no. Just the EH, or HS sites the tourists frequent, then? Perhaps. But that would imply that Skara Brae, Stonehenge, Avebury are more important than Soussons Common, or the Nine Stanes (having fires in their cists) which tourists can't be arsed to see? Or than a fine Bronze Age cairn hacked around by ignorant walkers... over and over and over again. Which. Must out of sight be out of mind?
I would say increased security isn't the answer. Increased awareness and community policing, however, may perhaps help. Signage could help also. Shame those that can be shamed into respecting what they may have considered harming. But excessive fencing will - in my opinion - 'destroy what it is meant to protect'. Monuments have to be 'used' to live, to be relevant. If I can't have access I will not go.