On the Yahoo answers site there's this reply
"... If you're sure you want to delete your account, make sure you�re logged into the account you wish to delete and then click on this link: https://edit.yahoo.com/config/delete_use...
Yahoo will ask you to enter your password one last time and then you will click on the "Yes, terminate this account" button. Your entire account will then be completely & irretrievably deleted. "