In your Online Control Panel
Click Exchange Hosting Service
Click Hosted Organizations
Click your domain name
Click Mailboxes
Click the Red "X" to the right of the mailbox and confirm deletion.
This only removed the mailbox and not the existing user account. To remove the additional user account:
In your Online Control Panel
Click Exchange Hosting Service
Click Hosted Organizations
Click your domain name
Click Users
Click the Red "X" to the right of the mailbox and confirm deletion.
