Although username accounts are not deleted, so you cannot be impersonated and have your reputation affected by anyone registering the same username in the future, there are steps you can take that amount to removing all personal information and result in leaving only a throwaway email address behind. http://en.support.wordpress.com/deleting-accounts/

1. You can go to My Account->Edit Profile and remove all data except the account name and an email address. Create a throwaway email address and change the account email to that throwaway address. This only leaves the username behind.

2. To unsubscribe from email sent to you from WordPress.com, go here and click on “Block all subscription emails from WordPress.com”:
http://wordpress.com/#!/following/subsettings/

3. To ensure the account is locked up tightly, visit this site, generate a password of 15-20 characters, and change the account password to it:
http://www.thebitmill.com/tools/password.html

4. While you cannot completely delete your Gravatar profile, you can remove all of your profile information easily through the "Edit My Profile" page here:
http://en.gravatar.com/profiles/edit/?noclose#about-you

You can also hide your Gravatar profile from external access. Visit your profile page (click your name in the top navigation of the website), then click the "Hide My Profile" link in the bottom, right-hand corner.