Issuing the F/As in J with Tasers might be an option but that could take away from the service. "Excuse me sir, your champagne will be a moment while I attend to an interloper. You might consider putting on your headphones to mute the screams. I'll be right back."

Perhaps the answer is an invisible fence. Fit every Y passenger with an ankle bracelet that will cause an excruciating electric shock if they wander past the threshold into J.