So you're saying you wouldn't notice that a province is being logged on to from a kingdom mate's IP?
You might not be able to stop them before it happens but is the risk of getting both provinces deleted afterwards not a good enough deterrent?
I agree that this is possible for maybe the first 24 hours of war but once people have been chained, you might find yourself with no targets in your range because you can't control who will be chained by your opponents.