My Suggestion:
1. Take him into confidence. Tell him it hurts and harms the company protocol. If he listens, you can guide him into better organizational behavior.
2. Find out if he has some conflict with someone inside with who he addresses issues relating to his profession, salary and existence in the company. He might carry a personal grudge against that person; by doing such a thing as you state here, he might think that he is taking revenge on that person or system. Is he happy with that person, system and himself? - find this out and then try providing solution - directly or through other channel as fitting. My feeling, it might happen to be you yourself, in case he is reporting to you and you have an issue with him. Sorry, its only a feeling and a suggestion.
1. Take him into confidence. Tell him it hurts and harms the company protocol. If he listens, you can guide him into better organizational behavior.
2. Find out if he has some conflict with someone inside with who he addresses issues relating to his profession, salary and existence in the company. He might carry a personal grudge against that person; by doing such a thing as you state here, he might think that he is taking revenge on that person or system. Is he happy with that person, system and himself? - find this out and then try providing solution - directly or through other channel as fitting. My feeling, it might happen to be you yourself, in case he is reporting to you and you have an issue with him. Sorry, its only a feeling and a suggestion.