It sounds simplistic, but you can just tell your bully to stop. There are no guarantees that they will cut out the bad behavior, but there’s a good chance that a coworker who’s reasonable in other ways might just not realize that they’re upsetting you. Plan your speech ahead of time, be clear about what you want them to change, and make sure you give them space to respond.

The good news is that you have a window of opportunity to nip things in the bud, before you become the long-term target of a workplace bully. Here are few options:

• Call attention to their values: Try “I know that you really care about everyone feeling valued, and when you do X, it undermines that intention. Maybe we could try Y in the future?”
• Explain why it’s a problem: Try “I notice you X, and when you do that it makes it hard for us to foster a team environment.”
• Say their name a lot: Try “Jim, I hear what you are saying, but Jim, I need you to stop doing X. I treat you with respect, Jim, and I need you to do the same.”

And don’t forget your body language. “Stand up tall, arms at your side, nose up”. “If you’re feeling nervous about standing up it will show through with arms folded, shoulders hunched, looking down.”

The bad news is that if you brush off bullying and let it continue in its early stages, it’ll only get worse. “A lot of times people let it go and let it go and let it go,”. And by the time they realize they’re being bullied, it might be too late. Once that power imbalance has been cemented, it can be virtually impossible for the target to fix.

In other words, if you muster the courage to speak up after months of being bullied, the abuse is not just unlikely to stop, it may even intensify. So if you’re that far down the path, you might be better off taking a different approach.

Reference: https://www.themuse.com/advice/how-to-deal-with-workplace-bullies