My thinking would be that it would be a fairly large radius where the very edge would minimally increase the chance, with that increasing the closer inside the zone you are. You’re right that the purge meter only goes up upon building and a few other actions, however perhaps its possible that structures within these zones could be checked for at server restart and count as newly placed, or something to that effect.
I get that people will just build in that area +1 which is why I think it should be a wider radius with a minor effect if you build within the edges, but a major effect the closer you are. That way you could still build nearby if you like, but you assume that extra risk, yet walling the whole place off without assuming said risk would be a tremendous undertaking. You’re right that it’s probably too much effort to be worth though.
Without a solution though you’re just going to see more players driven away from the game, people that don’t want to start entirely over because some kids with more time than God wall off the entire map. Short of hiring a moderator team for official servers I don’t see how else this could be handled, without some kind of in-game mechanic to do it automatically.