How Can I Stop Employees Using Their Smartphone Provider S Internet Service On Company Grounds

My question would best be described by the situation a friend of mine finds himself in:

A friend of mine has a company and he finds that his employees are using their smartphones too much, i.e. the employees are not using the company’s wireless signal but the Internet on their smartphones from their service providers. According my friend he didn’t mind in the beginning but now he finds that they are on their smartphones too much.

So I’m asking does anyone know if there is there software or hardware available that can stop the employees from using their service provider’s Internet on their smartphones in the company grounds?