Verified answer

Make sure you have signed out of any account you wish to keep and then click here:

https://edit.yahoo.com/config/delete_user

I recommend you read the following warnings from Yahoo:

When you terminate your account you will lose access to your Yahoo! ID, Yahoo! Mail, and profile names. They will also delete your account data and settings across the Yahoo! network. This includes any contents in: Yahoo! Mail, Yahoo! Address Book, Flickr, Yahoo! HotJobs, GeoCities, Yahoo! Briefcase, My Yahoo!, Yahoo! Groups, Finance Portfolio, and any other area of Yahoo! that can only be accessed with your Yahoo! ID and password.

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