Hi @Anonymous ,
Welcome to Microsoft Q&A forum and posting your question here.
I have a user that is showing mailbox accounts no matter creating a new Outlook profiles with just a single account.
From your decripiton, it seems that you add an account in a profile, but other accounts are also displayed, right?
According to my test, the reason for this situation may be due to the user was granted with "Full Access" permissiom.
When it automatically appears under the user's mailbox, Exchange administrator can remove the permission in the EAC:
(1) In the EAC, navigate to Recipients > Mailboxes.
(2) Select the two default mailbox.
(3) Click More options. Under the Mailbox Delegation option, selecting the user to remove from the existing delegates, click Save.
Here is an official document which contains detailed example for reference.
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