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.
Deleting All Data for a User
From the navigation pane, go to Protect > Office 365.
The Office 365 apps page appears.
In the table, click the app that contains the mailbox.
The app page appears.
Remove a mailbox from backups or delete data based on the following requirements:
To exclude a mailbox from further backup operations, in the row for the mailbox, click the action button
, and then select Manage > Remove from content. The status of these mailboxes is updated as Deleted.To permanently delete all the data that has been backed up, in the row for the mailbox, click the action button
, and then click Manage > Delete backup data. The data is no longer available for browsing and recovery. A confirmation dialog box appears.
Click Yes.
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.
Deleting Mailboxes, Folders, and Messages
From the navigation pane, go to Protect > Office 365.
The Office 365 apps page appears.
On the Apps tab, click the app.
The Overview page of the app appears.
In the table, click the app that contains the mailbox.
The app page appears.
On the Mailboxes tab, in the row for the mailbox that contains the backed-up content that you want to delete, click the Actions button
, and then select Restore > Restore messages. The mailbox contents appear.
Select the mailbox, folders, or messages that you want to delete:
To delete a mailbox, in the left pane, select the mailbox, and then at the top of the page, click Delete.
To delete a folder or a sub-folder, in the left pane, expand the mailbox and/or sub-folders, select the folders you want to delete, and then at the top of the page, click Delete.
To delete messages, either expand folders in the left pane to locate messages in the right pane, or in the Search box, enter search terms to find the content you want to delete, and then at the top of the page, click Delete.
The Confirm delete dialog box appears.
In the box, type DELETE, and then click Delete.