♦You can delete all your E-mails in one go if your using “New Yahoo Mail” click “More Action”>”Select all messages”>then click Delete and your done. If your using "Yahoo Mail Classic you can only delete one page at a time. Increase the number of Emails displayed per page to get more at a time and delete them totally.
1. Click Options, ‘Mail Options’ at the top right of your Yahoo Mail page.
2. Click on ‘General Preferences’.
3. Go to the "Message Results per page section.
4. Click the Blue down arrow and select 200 (maximum).
5. Go to the bottom of the page and click ‘Save’.
♦Go back to your In-Box page and scroll down to the bottom of the first page. Click "Check All" (this will place a check mark in all of the Emails on this page) then click the Delete button. Go to each page and repeat this process. Click Empty next to your Trash folder if you are sure. (http://help.yahoo.com/tutorials/cg/mail/…