If, and that's a big IF, HAL really wanted to address this problem, there is a simple way to stop all cell traffic in a given area. A sign won't stop it, but a
"Blocker" will.
A cell blocker is an electronic device that does just that. It blocks all cell signals in a given area. While this would not be allowed under FCC (Federal Communications Commission) rules in shoreside restaurants, a foreign flagged ship in international waters would not fall under FCC regulations.
Problem solved.