How to get an angry person to calm down:

Two experiments were carried out to explore an anger-reducing strategy based on Brehm’s emotional intensity theory. According to this theory, anger can be reduced indirectly by interfering with the feeling of anger rather than by dealing directly with the source of anger. One strategy involves providing the angered person with a reason for feeling happy. We predicted that anger intensity would be reduced not only by a large reason for feeling happy, such as a large gift, but also by a small reason, like a tiny gift. A medium-size gift was expected to maintain anger at approximately its instigated level. Both experiments instigated anger by personal insult and then measured the intensity of felt anger and retaliation after either no further treatment, or a small, a moderate, or large irrelevant gift was presented. The results for felt anger and retaliation confirmed our theoretical expectations.

Source: “Deterring Anger and Anger-Motivated Behaviors” from Basic and Applied Social Psychology, Volume 30, Issue 4 October 2008 , pages 326 – 338

Knowing that you need to deal with the emotions of anger vs the source is a powerful tip.