Before you can remove mailbox owners, you must search for them to list them in the Search Results section of the User Search page. You can use a Quick Search to find a specific user by name, or you can create an Advanced Search that locates a particular group of users.
User > User Search
Execute a search for the mailbox or mailboxes you want to delete.
In the Search Results list, click the empty box next to each user you want to delete.
To select all users listed,
click the empty box in the column header.
Click the Delete Selected button.