Please follow these steps to deactivate the auto-renewal for the mailbox:
1. In your account go to PRODUCTS > EMAIL
2. In the list of domains, click on the "View" icon to the right of the concerned domain and then on MANAGE to the right of the concerned mailbox.
3. Set the "Mailbox auto-renew" option to OFF.
IMPORTANT: If you deactivate the auto-renewal option, you will have to renew it manually before the expiration date to avoid permanently loosing all of your mailbox's contents.