Client Wants me to "Block Wifi" in the bathroom

Hi All,

Let me know if there is a better place to post this question.

I have a client who is having ton of issues with their workers hiding in their bathroom and playing on social media. They feel it is a grey area to go in and tell them to get out of the bathroom. They are requesting we install a signal jammer in the bathroom to stop the issue (which I believe is illegal in the U.S.?). My first idea was content filter or some firewall rules but that does not stop them from using their phone carrier 4G connection to get to social media. Is that the best solution I can offer them? Any ideas? Right now the prevailing solution is to only allow personal devices on the guest network and turn off the guest network during work hours(enable on break), or install a content filter without a solution for the cellular/4G side of things.

Thanks for any ideas! I am willing to work outside the box on this one!

Edit: So I am going to stand by my original answer to them, and only provide options for content filtering on the wireless. Thanks ladies and gents!