If you're certain there is no mail to be kept in the old account it can be deleted.

Press Alt or F10 in case the menu bar is hidden. From the main menu bar: Tools - Account Settings Select the account to be deleted in the left pane. Select 'Remove Account' from the 'Account Action' drop-down menu at the bottom.

I do not have the option to "remove account". I followed the directions, but something seems to have changed since your answer.