You can delete a mailbox from an Exchange Online app.
If you added the Active Directory group to the Exchange Online app, and the mailbox is included in the Active Directory group, then the mailbox will be added back to the app when the next backup runs.
From the navigation pane, go to Protect > Applications > Office 365.
The Office 365 apps page appears.
In the Name column, click the app that contains the mailbox.
The app page appears.
In the row for the mailbox, click the action button , and then click Manage > Remove from content.
A confirmation dialog box appears.
Tip: To see deleted mailboxes, on the Mailboxes tab, click the gear button , and then select Clear all filters. In the table heading row, click any Column Settings button , and then select Columns > Status. The Status column is added to the table. Deleted mailboxes have a status of Deleted.