D.B.
I'm a stepmom of 2, with my husband's very difficult and impossible ex.
My advice is: don' try to counter her BS. You should block her. She has no reason to contact you. Her contact re the children should be your husband. Period.
Your husband should no block hert. Cutting off the mother of the children is a truly bad idea, and will make it look like he is cutting her out of the picture. It won't work. Judges don't like that, and it shows that shared custody doesn't work - which makes judges void it. Don't do that. He should save every stinking text she sends, and be extremely careful about what he says in reply.
Here's the horribly hard part: do not engage the children in this. Do not try to talk about it or say that Mom is wrong. It will backfire, and they will report back to her because they are kids and love their mom. There is no discussion with them about what she says. None. It's horrible. Do it anyway. Find a couple of standard phrases like "I'm sorry to hear that" or "I have a different view on that" and then break out the Monopoly game or the stuff to bake cookies. You don't have to go to every fight you're invited to. If they fight you on it, just bake the cookies. Draw the hopscotch board n the sidewalk. Plant the mums. Go shopping for pumpkins. Anything. If they bring it up, you say, "These are parenting discussions to have with your dad and mom. I'm not interfering. I love you. Let's cook/plant/bake/shop." That's all.
You're in this for the long haul. You may not see results until the kids are 18. That's okay. Your job is to show them a good marriage, a mature and calm woman, and a "big picture" person. My stepdaughters have their problems, including domestic violence and worse (as if there's anything worse...) But they absolutely know that my husband and I are the model of a supportive marriage. You do what you can. Moreover, your own children are learning about how to stand strong in the face of onslaughts and unfair statements. Teach them not to react to everything that goes on around them.