set up a code of conduct:
i.e. tell them they can surf/check email
for 1/2 an hour at lunch, maybe the start of day and the end of day, that
way they have the best of both worlds, any one abuses the system face the
punishment,, 3 strikes and your out, style rule,, check phone
bills/logs etc find out who is dialing /when /what to /how long etc
If you stop it totally you will Pee ppl off, so meet them half way!