I'm going to ask my boyfriend to kick his ex-girlfriend off of his netflix account
Why are you going to ask him to do that?
Its an ex she doesn't need to have contact with him anymore.
When a relationship ends people are expected to move on and separate belongings. You should tell him first and get him to change the password so you don't cause any relationship conflicts.
What if he is loyal to her? It's not like he has any reason to do that.
It is understandable to not share any kind of media accounts with your ex. He may not be aware that she is still using it since he doesn't think about it. I think you could bring it up politely and ask him if she is still using it.