For me, whether it precludes someone changing or not is irrelevant -- I agree, @pikapikachu, that often your best option is to stop having any contact with the person, and yes, there are depressing numbers of posts on Mn where people tiptoe around enabling other people's appalling behaviour. For me, the problem is that you can't legislate for how that person is perceived by and interacts with other people.
My grandmother's relationship with me, her eldest grandchild, was toxic and very damaging, and her relationship with her three children was toxic in life-alteringly damaging ways I can see being passed on to the next generation -- but my mother and uncle, for instance, would not recognise my description at all. For them she was a beloved mother. For my younger sister, she was a lovely, warm granny figure.
Do I conclude that I'm right and they're wrong? That my younger sister was simply too young to understand the dynamics, or that I came in for most grief because being the eldest girl, that my grandmother was casting me in the role of dutiful handmaiden, like she had my mother? That my mother and uncle don't have enough self-knowledge to understand how damaging she was to them, and to me?
It's messy and complex, like most human relationships.