How does she get her to stop posting photos of her kids?

“I care a lot about my kid’s privacy and I don’t post any photos with them online. I have three kids with my ex husband. His new girlfriend has a public profile and posts photos of our kids on her Instagram. I told her I didn’t want her posting them and she blocked me. My ex husband says it’s not up to him what she posts on social media. What should I do?”
Pete KellyWeekdays 3-7PM