I get that things were / are difficult with your ex but that's your issue, not your daughters. I repeat. YOUR issue, not your daughters.
So ask yourself this question. How is talking badly about her father serving you well? It isn't, is it. It's time to stop. She's told you how she feels and frankly you're hurting her.
I get putting boundaries in, like not having him in your yard. That's more than fair.
But if you're wanting to say anything negative about her father again, ask yourself if it's necessary to tell her.
What was your intention of even telling her in the first place?