There are one of two real issues here:

1. Is the employee accomplishing sufficient amounts or high enough quality in his work efforts?

If not, deal with this situation on that level as a performance matter--advise him of his shortcomings, counsel and discipline (up to and including termination of employment).

2. The employee is doing a great job but you are annoyed by him breaking a company policy (showing at the very least some level of dishonesty).

If this is the case, presumably the employee has a supervisor. So, the old-fashioned but effect use of observation comes to mind. I would not go further than that.

One of these two ways will work, if applied.