The best thing to do is get up really close to the person and say clearly, “Calm down. You’re getting excited. It’s not a big deal. Calm down”. It is best to use calming hand gestures at the same time (arms out, palms facing downward, vertical motions).
If you are wearing protective gear (or if the person is significantly smaller than you), you can probably repeat this 3-4 times before you lose consciousness.