Start a new email and on the right side at the top of the stationary selection click "none".
To avoid this problem I compose my emails as plain text and hide the stationary.
Here is a video showing the process the make both of these changes :
[ame="http://www.youtube.com/watch?v=bGwcC9bZJco"]YouTube - How to disable stationary in Yahoo Mail.avi[/ame]
- Start a new email
- Click show stationary
- Click none under the stationary
- Click plain text to compose emails using regular text