This is something I experienced before and expected to happen after upgrading to version 1.1.8.
I don't want all my posts to show a featured image, but the Montezuma theme unfortunatly likes a featured image a lot and will show some other image from the post anyway. So I had to change some code again to fix it.

Since the code has changed slightly, I thought it would be nice to post the solution again.

Open thumb.php and change TRUE to FALSE in line 36 and 41:

elseif ( FALSE !== ( $thumb = bfa_get_first_attachment_id() ) )
$hasthumb = FALSE;
elseif ( FALSE !== ( $thumb = bfa_get_first_unattached_gallery_img_id() ) )
$hasthumb = TRUE;
// if local image not added with WP uploader but added as manual HTML link
elseif( FALSE !== ( $thumb = bfa_get_first_img_src() ) )
$hassrc = FALSE;

This way the smart featured image handling is disabled.

After "upgrading" to version 1.1.9 it looks this fix doesn't work anymore. Groan.

Any idea how to make changes to thumb.php now to not include the featured image?

The alternative way, to delete the line of bfa_thumb code from Montezuma postformat.php seems too aggressive and is more of an all or nothing approach to featured images. Sometimes you want to include a featured image in a post and making the changes to thumb.php worked well before to give you that option.

I just did the upgrade and changed thumb.php the same way as I did for 1.1.8. To check the change I force a refresh by updating one of the posts. All worked well for me.

By the way, I use a childtheme for customizations: Add Customized Files “/Includes” Folder to WordPress Child Theme