Dear Diana Barreneche
Thanks for raising your concerns in Microsoft Community.
According to your description, I understand that you have subscribed to Microsoft 365 Family and want to cancel the sharing of an old account. I understand your feelings and this problem is indeed troubling.
First, you need to check the sharing list in the account to see if the old account still exists: you need to visit [Subscription], the interface will display Microsoft 365, and the sharing list will be listed to check if there is an old account. If so, click [Cancel sharing] next to the user name to activate it.
After cancellation, the old account will no longer have a Microsoft 365 license.
If Office still recognizes the old account, you need to check whether the old account is logged in on this computer: In the computer's [Settings] > [Accounts], check [Email and Accounts], [Other Users], and [Sign in to Work or School Accounts] to see if the old account exists. If it exists, you need to delete it.
You can then log out of your account in Office and log in again with a new account to see if Office can recognize it successfully.
If the problem persists, can you take a screenshot of the [Account] interface in Office and share it with us through a reply? This will help us clarify the specific circumstances of the problem.
I hope the above information is helpful to you. We are looking forward to hearing from you and will continue to assist you.
Best Regards,
Tracy - MSFT | Microsoft Community Support Specialist