I just did, it was a super old account that was filled with anything but spam emails.

Preparation: Make sure that no email that you need to keep in the account and backup your address book.

Now the process, which is as simple as this:

1. Go to https://edit.yahoo.com/config/delete_user and login use the account that you want to delete.
2. Fill in the required information including your login password and a case sensitive spam captcha (quite confusing sometimes).
3. Done.

From the deletion request page, the deletion of the account will be finished for about 90 days or less, although as soon as you push the delete button, the account is automatically deactivated.