Yahoo! provides you the facility to terminate your account whenever you want.
The process goes like this, once you complete the termination process (which is pretty simple) your account gets deactivated and scheduled for deletion in 90 days.
If you want to terminate or close your Yahoo! account simply goto the following page and follow the simple steps to deactivate / delete your Yahoo account here: Yahoo! account termination page.
Yahoo! will ask you to sign in or verify your account password if you are already signed in before you continue to the account deletion page.
Although the account deletion process is pretty simple, but still make sure you read all the information provided on the page before you continue. Once you delete your account you can’t reactivate it and you lose access to Yahoo! ID, Yahoo! Mail and profile names, deleting your account data and settings all across the Yahoo! network. However it will not delete your Yahoo people search listing.