he should signal you that he is leaving the room and go to a prearranged spot to calm down. Let him know that that is not a punishment, but a way of helping him calm down. Explain that he can return when he is feeling more in control. Some possible cooling...
“Because I’m the GM and I said so”. Actually, I guess that’s the second step. The first step is to run a good enough game that people will play it even if you use yourparent voiceand make them track inventory because you’re the GM and you said so. ...