Can I cancel/delete '''all''' my Canonical/Ubuntu accounts?
Yes, but you will need to be careful to delete the accounts in the correct order. The general rule is to delete each of your OpenID consumers first, one at a time, and then delete your SSO account last.
OpenID consumers include services like Launchpad, Ubuntu One, Landscape, the Ubuntu wiki, and third-party sites such as AskUbuntu. If you have used any of these, you should delete your accounts there before having your SSO account deleted.