Mail Format requesting employees to not step out of the office during working hours

Off late, I have noticed that the employees in my office have been stepping out of the office on different pretexts- to smoke, buying snack, personal work etc.)
I am looking for a format of a letter I could write instructing/ requesting the employees to refrain from carrying on with such practice. Ours is a corporate office with less than 50 employees and no security guardrequest

Dear friend,
Are you empowered to take a decision in this respect? What nature of work you people are doing in your office, only admin.?
Do you have an official tea time schedule during working hours? If not consider imposing one say with 10 minutes break. And for any violation, issue individual memo with a warning of cutting half a day leave. Check any clause in your Standing Order, if any in vogue. If not draft out one model SO and introduce where such matters could find place.