Please note that WordPress.com accounts are permanent. You can find more info about that here:
If you are trying to unsubscribe from emails sent to you from WordPress.com, go to Email Delivery Settings and click on "Block Emails":
Here's a few suggestions for you if you'd like to scrub the account:
1) You can also go here to 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 will only leave the username behind:
2) 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:
3) While you cannot completely delete your Gravatar profile, you can remove all of your profile information easily through the "Edit My Profile" page here:
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.