Verified answer

I'm pretty sure you can do it from My Account, if not then just get yourself banned it might not be the most pleasant experience but it could be fun and they would delete your account that way, but please dont do that unless theres no other way, and there should be

To delete your Yahoo! account, go to: https://edit.yahoo.com/config/delete_user

Your account will also be automatically deleted if you don't sign in for approximately 4 months.