Closing a Yahoo account deletes Yahoo! Mail, Address Book, Flickr and other services linked by that Yahoo! id. Here are the steps to close it -
2. Sign-in with your id and password.
3. Confirm your password again; verify the captcha code and hit the Terminate this Account button.
Note: If you have subscribed to Premium services, you'll have to cancel each subscription before deleting the account. You can do that by visiting the https://billing.yahoo.com/ page to cancel subscriptions.
How to close Google Account?