After clicking "Forward" meassage, place flashing cursor in front of the first old email address at the top left corner of the message page. Hold down the left mouse button and drag across and down to highlight all the old addresses. When all the old addresses are highlighted hit the deleted key on your keyboard.
Sometimes they won't delete using this procedure and you have to use the same process for a few at a time. For the long winded method, put you cursor at the very beginning of the address and keep your finger on the delete key until they're all gone.