The site doesn't provide an explicit method for closing a user account.
However, since we don't use your data for anything (we don't spam and we don't sell or share email addresses), we don't benefit from keeping your account indefinitely. So, in order to save space in our database, we regularly delete inactive accounts. Any account that hasn't been used in one year gets deleted from the database. After three months of inactivity we send a gentle reminder, then at six months we send a warning that the account will get deleted, and then at the one year mark, we do delete it.
The exception for this rule is that premier accounts (even expired ones) get deleted after 7 years. And user accounts with pen names and stories don't ever get deleted.
If for some reason it's important to you, we can delete your account for you manually. Use the 'Webmaster' link to request account closure.