Firing employees for job hunting is a mistake on so many levels.
1. It signals insecurity about your culture and why anyone should want to work there. (We can't keep people here except under threat.)
2. It's ineffective against your best employees, who will have options and will not fear you.
3. It demotivates everyone and creates a vicious cycle of fear and distrust, which will drive a culture of keeping your head down and hoarding information, which will destroy the efficiency of your remaining workforce.