I don't want to tell you how to run your business, but have you had a talk with your employees about what is acceptable/responsible internet use and what is not?
Personally I think overuse/misuse is one thing but like a quick call to or from home sometimes an email might be OK.
Maybe a meeting where rules are laid out would save some animosity when Joe Worker comes in Monday AM and all of a sudden can't check his personal email anymore.
