Unfortunately, a while back, Yahoo changed how multiple messages about the same subject (even to different reflectors - those are "bundled" into a single message) are handled and, when a reply to any of those messages is sent, all of the attached messages cannot be deleted! At least I have not found a way!
Try switching to "plain text" for that reply, and then switch back to 'rich text'. You can delete the unnecessary text. In the compose window in my browser, it's a couple up arrows in the gobbledygook next to the send button. Mouse over it and it will say 'Switch to plain text'. Do it again to reverse.
73 N4UFO
------------------------------------------------------------------ "Control is the need of the fearful mind. Trust is the need of the courageous heart."