If you are a kid, tell someone in authority and let him or her deal with the problem. If you are an adult, report to a supervisor. In the community, bullying is a police matter, call them. No one deserves to be bullied.
Try to remain calm and unintimidated.
If you are confronted, you can’t appear weak; waiting and hoping and quaking. Appearing confidence is your best defense. This may mean immediately confronting back. State your boundaries, clearly and loudly, “Back off” or “Enough” or “Go Away”. Then, breathe deeply and walk away. Let the bored bully find excitement and an easier target elsewhere