There is no one definitive answer to this question. It depends on the specific domain and the way in which you want to leave it. If you are leaving a domain as part of a merger or acquisition, you may need to consult with the new company's administrator to determine how to do this. If you are leaving a domain on your own, you may need to create a new account and password and then follow the instructions on the domain's website or in the help file to leave the domain without admin.