It's on a public footpath. It's legally quite hard to stop people walking on those whenever they want to. Also, the land owner is the National Trust, and they don't tend to want to restrict access to open spaces like this. Also, there's not very much left there to vandalise now - you'd get more benefit by restricting access to the trees that are still standing elsewhere in the entire country, which is never going to happen.
Are you from the area? I was under the impression this place is a vast open field, surrounded by farm land. Can’t imagine how you would restrict access.