This one's easy. She starts talking to a guy not including you in the conversation for hours. Break up with her anyway you like. Make it fun and entertaining for yourself in the least and most. If she's going to be a prick, then you're going to be an asshole. If you don't show her that you're a force not to be recokoned with, then that'll lead to problems later with you and her even after you're broken up.
I don't mean any kind of physical force here. Just whenever i break up with a girl, it either goes smooth or great. If it was a great relationship, it goes smooth and everyone has their respect in tact. If it doesn't go smooth, then that means i took my happy ass time being an asshole because she happened to be ****ed up.