How to remove toxic people from your life

I always read about eliminating toxic people from your life. What if the most toxic person in your life is the mother of your step children? What if you have no legal right to remove her from their life, even when you see her neglecting them and treating them like accessories or toys.

Loading them with treats and giving them back after an hour or so of having them around to show people. Expecting you to take them whenever a social opportunity comes up for her. Meanwhile she doesn't work or care for the children. We do everything. She is taking the child tax and not giving it to their primary caregivers. She just comes and goes and takes them as she pleases, leaving us to damage control when she disappears. And really I just want a bit of time to clean the mess of her junk she left behind.

How do you deal with someone like that? I just want to protect the kids but I have no right to do anything about it.