When you try to delete a user in SoftLayer (either via the portal or the API), the user is put into a 'cancel pending' state. They will not show up in the users list, and cannot login. However, you if you try to create a new user with the same email address, the system will render this message: This username already exists in the system.
How long does it take until the user is fully deleted and that email can be used again?