How can I remove a family member from Family Sharing?

I have followed all the steps from previous posts, spoken to Apple support, logged into my 12 year old Mac to try and remove an account from my family sharing... but NOTHING works. My dad's account has been locked out and he cannot access it anymore, and I want to remove it so I can add him. When trying to remove it from my Mac, it just hangs and nothing happens. Please can someone at Apple help me with this?!?

[Re-Titled by Moderator]