This is not at all the best way to go about this. Instead of this approach, a simple sign during peak hours near the register communicating a time limit will suffice. This approach is passive aggressive, hoping to influence behavior by taking away a feature. After all, this business is not having an issue with too much wifi usage, but with tables being occupied for too long so that is what they should address. Direct communication will also avoid constant questions about why the wifi is down.
Sometimes its how you say something that matters. Other times you shouldn’t say it at all. Serving the most customers as well as we can is the goal of every service business. Focusing on providing value is a better policy than publicizing how you limit it, even if the goal is to serve more people. Above all else, listen to how you communicate in your store or online as if you were the customer. Chances are that will filter how you say something or if you say or do it at all.