The following is from Yahoo Help.
To close or delete your Yahoo! account:
- Visit the Account Termination page.
- For enhanced security, enter the Yahoo! ID and password for the account you are deleting.
- If you have forgotten your password, please use our Yahoo! Password Helper.
- Please carefully read the important information facts in the Account Termination page.
- Confirm again your password in the field provided and enter the CAPTCHA code.
- Select the Terminate this Account button.
Go to http://help.yahoo.com/kb/index?locale=en_US&y=PROD_ACCT&page=content&id=SLN2044 before you close your account