There *is* a time, way, and place to tell a gamer to calm down and not get smacked silly.
Between loading screens or during saving mechanics, gamers aren't nearly as firey with their rage, and can be significantly more reasonable in that time frame. Just telling them to "calm down," however, isn't going to get you very far. Tell them to pause the game once the loading screen finishes, and to go take a break, maybe get a snack or a drink or something. While this might not work nearly as well for online games, telling them to take a break in between sessions or outings works for those as well.
As someone who has been pissed to the MOON at a game, having my boyfriend tell me to calm down and take a break significantly helped ease my distressed mental state.