They know the rules, everybody must be treated equally in the company. You ARE doing your job - even though it may not be pleasant all the time. Everybody is human and can "make a mistake" but thats what the procedures in the policies are for. You need to concider the safety point. If he is operating machinery and if something bad happends under the influence, everybody loses there! The employee, the company and especially you the manager. It is not easy thats for sure. Try a sexual harrassment issue! That's terrible too.