In Relationships where there is a child involved and the Relationship can longer exist, how should you treat your Ex?

Time and time again, typically we've seen Ex Wives or Ex Girlfriends make it a point to make the lives of their child's Father a living hell. They threaten to withhold their child from them, or bad mouth them in front of their child and "take them to the ringers" and as far as Child Support is concerned. But, there's a different way. One that allows the Ex to lead their relationship with their child. And there's so much freedom with it.