If you want to remove an Account, open its Settings page, and click the Remove Account button. The confirmation page should appear.
If you confirm the action, the selected Account, all its Mailboxes, Settings, and other Account-related data files will be permanently removed from the Server disks.
The Account Aliases and all Mailing List owned by this Account will be removed, too.
You cannot remove an Account when it is in use.
If you are a Domain Administrator, you should have the Can Create Accounts Access Right to remove Accounts from your Domain.