If you find yourself in the position of having to calm someone who is enraged, belligerent or out of control, begin by controlling your own reactions to the individual's behavior and realizing that the aggravated individual is probably just overwhelmed with anger and unsure of where to direct it. Anger management tools can be put to work talking the person back into a rational state.
How to Calm Someone Down
Express understanding for the enraged person and validate his feelings. This can go a long way toward disarming a volatile person.
Use a calm and relaxed voice when talking to the person without being condescending or sarcastic. Maintaining an even tone when talking will reduce the level of confrontation.
Maintain clear boundaries to establish mutual respect, safety and fairness. If those boundaries are already clear, politely reinforce them as necessary. If the angry person cannot abide by your boundaries, remove yourself from the situation and agree to speak to the individual later when he has calmed down on his own.
Flood them with kindness and compassion. Most people have a hard time screaming and yelling at someone who is demonstrating compassion and concern for them.
Forgive and forget. By showing forgiveness, you can maintain a relationship with the person and work through the conflict. While it's hard to forget seeing someone lose emotional control, reminding the individual about the incident might trigger another outburst in the future.