We do something similar with Katie. If she starts being a pest, she gets a time out. After the third out, we go home. She's youngish, so I'm hoping she'll learn how to be more polite.That's when you step in and stop him yourself. You saw him pestering Bear repeatedly after he didn't want to play anymore... you need to act and not risk a possible fight.
When I still went to the dog park my dog would get obnoxious with some dogs who didn't want to play with him sometimes. If he did I redirected him, if he continued we left. Don't allow your dog to be a jerk.