I enjoy the "friendly ahem reminder" approach first. I second sending an email blast or memo to the staff and reminding them where they can find the policy in their employee handbook.
Once you have sent the reminder monitor the situation and then start exercising disciplinary forms. Make them understand how serious this situation is by having the employees sign a write of of some sort. Explain the process and you may even have to implement a probationary period in which if the same problems happens X amount of times during X amounts of months their employment would be terminated.
Many times it can be an honest mistake and the honest ones will get the hint after the email or the first discipline form is signed. Also, you now have evidence on why this employee was terminates should you end up defending yourself in court or EEOC.