The best way to stop bullying is to make sure that someone else knows about what's happening to you. Too many victims suffer in silence, so tell a friend or adult who you trust to defend you against the bully. If you tell a someone about the situation and they don't take you seriously, then tell someone else. Keep talking about it until someone is willing to help you resolve the situation.
If this is happening at work, tell that person's boss what they're doing and talk to your HR manager. If the bullying has escalated to the point that you are being physically harmed, don't hesitate to contact a lawyer and talk to police or other authorities.