I am managing a 20 member team. My best employee is currently looking for another job. What is worse is that he has completely lost interest in working and is now using office hours to prepare for job interviews with 3 other employees. They seem to be using up to half of the time they are in the office preparing for job interviews. I have been noticing this behavior, but not commenting (yet). Recently, the above employee lied to apply for medical leave, and was granted the leave. I have confirmed that the reason for the leave was a lie. This leave period is more than one month, which he intends to use to prepare for interviews (from what I can tell).
How can I avoid other people following his way of using employee privileges and other office resource for these kind of activities?