You have to login to your accounts control panel at: http://{forumurl}/acp and click on "Delete Account" and then fill in the correct information to delete your account

But if you prefer just to put your forum offline for a while, it's better if you deactivate your forum under Applications.

Please remember that we can't help you to delete accounts manually. If you can't login to the adminstration panel, you can always reset your password with the same username or email that you used when you first registered your forum.