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. Click the big red 'Delete' button at the top right.

There is no big red Delete button.

Select your account on the left pane, then click the Account Actions button on the bottom left corner, then select Remove Account. That should be it.

There is no big red Delete button.

Then you're probably running an older version. Next time please mention the version of Thunderbird in your OP.