Please note that all your data will be deleted, and there is absolutely no recovery from that. Once your account is scheduled for deletion, it's gone for good. We repeat - we cannot restore any accounts that have queued for deletion.
To delete your account, you must first be logged into the account you want to delete on the web and then go to http://untappd.com/account and scroll down to the bottom of the page and click "Delete Account":
After you click this link, a modal will pop-up. Simply enter your password to delete your account.