Log into the Customer Control Panel with your main account's username and password.
Click on the "Mail" tab.
Select the mailbox in question.
Click on the "Storage" tab.
Click on the "Edit" button.
Uncheck the box the right of the "Storage Enable" section.
Click "Submit", and you've turned off storage and email will just get forwarded to your other email box.
Note: If there is email in the box and you want to delete it all, click the "Storage" tab again, then you see the "Delelte" button.
Click that button all the existing email in that box will be removed permanently off the server.