Well no, the door wouldn't open. But that's not the point, the person is actively attempting to endanger the aircraft, himself, his fellow passengers and crew. Someone with that mentality needs to be stopped, otherwise they may move from trying to open the door onto something that really could cause harm.
Not to mention (or underestimate) the effects it has on other passengers, who would definitely not want to fly with your airline again if you sat and did nothing. And if I were the Captain onboard I'd want you to do something about it.
