How to I get my gf to stop doing this?

I've been with my girlfriend for a few months. Our relationship is wonderful, except for one thing. Since we began dating exclusively, other men in her life interested in her began treating her poorly. I told her these other men need to respect her decision and treat her with dignity, or she needs to cut them out of her life before they become a burden. She agreed. However, she continues to respond to their rude text messages and phone calls in my presence, and she ends up in a bad mood which is a major buzzkill. To make matters worse, she has known some of these men her entire life, and seems unwilling to establish boundaries for their behavior. My girlfriend tells me these men even slander me though I have never met them, then turns around and calls me selfish for asking her to ignore these hurtful people. Any advice?