I think you want to stop the discussion because you don't want to elaborate on the who and why.
If you don't think it's black and white, then why not explain your more nuanced greyish view?
You might educate a few other people even if you can't change poor Nigel's mind.
For the record, the very question of "who and why" is a leading, straw-man argument. It suggests the need for further definition and rules, which is entirely contrary to the point I was making - as Nigel well knows. I'm sure, for example, that there are people already visiting Skara Brae without permission. The world hasn't come to an end. Thousands of visitors haven't started flocking there after dark. The monument hasn't been damaged. By all means, debate amongst yourselves "who and why". Personally, I'm quite happy with the status quo, where rules aren't rigidly and blindly adhered to.