It doesn't matter how great your point is, as soon as you lose your cool then you have given the student power. There is no chance of "winning" for you at this point.
Stay calm. Enforce consequences if you need to without adding anger as a part of it. When students look like they want to get you engaged in an argument, do the opposite of what seems natural and get even more calm than usual.
When you are calm, you think more clearly. Let the student know that you will talk calmly or not at all. You will feel much better about yourself.