Help! How do I handle my ex-mother-in-law?

During my weekends, she interferes with my disciplining my teenage daughters. If I ground them, or if we even have a minor disagreement, she will be over at my house and pick them up!! In truth, they'll call her or text her and ask her to, but only to get out of punishment. This is completely undermining me. She is trying to drive a wedge between me and my children. I have asked their father to intervene, but he won't. He says she has his permission to pick up the girls anytime.