Disable user’s account
Learn how to suspend user’s mailbox of a former employee or unused mailbox
IMPORTANT::: THIS WILL FULLY DELETE THE ACCOUNT WITH ALL OF ITS DATA, THIS PROCEDURE CANNOT BE UNDONE.
1) From Mailboxes page, click on the user that you want to edit.
2) On the tab General, scroll down to check list of options, and check Disable User.
3) Click save Changes or Save Changes and Exit.