Go to your Reddit preferences and select the Disable tab at the top of the screen. Follow the on-screen instructions to deactivate your account. You can also click on this link. You need to know your account password to deactivate your account. If you do not know, if it is not stored anywhere, you can do nothing. Administrators can not help you.
If your account is deactivated, no one can use that username again.Usernames cannot be used and accounts cannot be regenerated.