How to remove your Digg account
As can be read on http://about.digg.com/faq it is impossible to automatically delete your account. You should ask them using the webform named on this page.
Direct removal link: http://about.digg.com/contact
Helpful Trick: Removing online data
Helpful Trick: Remove third-party logins
If you signed up for Digg using a third-party login such as Facebook or Twitter, you probably want to revoke all access from Digg. Click here to find out how.