How to remove your Ubuntu One account
Use their webform to request removal, right here: https://one.ubuntu.com/help/contact
For more info (including paying members), read: https://one.ubuntu.com/help/faq/how-can-i-delete-my-account/
Helpful Trick: Removing online data
Helpful Trick: Remove third-party logins
If you signed up for Ubuntu One using a third-party login such as Facebook or Twitter, you probably want to revoke all access from Ubuntu One. Click here to find out how.