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”:
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: