How to delete a Gravatar account?
The description of the service:
Gravatar a portmanteau of globally recognized avatar is a service for providing globally unique avatars. On Gravatar, users can register an account based on their email address, and upload a digital avatar to be associated with the account. Gravatar plugins are available for popular blogging software; when the user posts a comment on such a blog that requires an email address, the blogging software checks whether that email address has an associated avatar at Gravatar.
You can't delete your Gravatar Account without deleting your entire WordPress Account.