Can we resolve an office problem without punishing everyone?

The office has 19 employees, and most have cell phones they can use at their leisure. The problem is two employees are severely abusing the privilege by spending several hours every day on the phone when they should be working. Several people have tried to talk to them about this, but they insist there are no office rules restricting cell phone use.

If it is left up to management, they will make a rule of “No cell phone use during office hours” which will punish everyone, even those 17 who are not abusing the privilege. I would like to find a reasonable way to solve the problem without an all out ban on cell phones.