Just cut your losses, remove it from mail.app and check it via the interwebz browzer!
Seriously though, closest you will come is going to preferences, choose accounts, click the account you wish to hide, go to the advanced tab and uncheck enable this account.
The account will not be in the main mail.app window, but it will still be visible in the accounts portion of the preferences.
Otherwise, you'll have to use the method from my first statement. good luck.