I find, if I can't talk to someone about it, to write it down and then throw it away.
You seem to be indicating that it is a situation where you don't have the chance to walk away or to take a time out. In that case, take 5 deep, slow breaths.
Something I was told, and occassionally does work for me, is to smile in the face of these situations. If faced with an irate colleague or customer, if you smile, it prompts your body to relax. It can infuriate people further but it can also relax them and make them easier to deal with.
I hope this helps in some way!