How do you stop an ex from interfering in our marriage?

My spouses ex knows how to push buttons. My spouse will speak to me about the problems and when I agree or give my opinion, that's when my spouse thinks I am bias. Somehow the ex involves the child which makes the problem more complex. This is not good for our marriage.How do you stop an ex from interfering in our marriage?
Forget the ex, do what you know is best unless there is a court order that says otherwise. I used to be a kid in a custody fight, I know; Do what is best for the kids, they will love you for it when they get older.