How to delete domain or account on website?

We are looking to offboard with a client, and we are trying to regain ownership of a limit login attempts account that they are no longer using, but is using one of our emails. After logging in, the interface shows no option to remove a domain or to delete the account. How do we do this from the official website side, and not from the website it’s used on?

The page I need help with: [log in to see the link]