Help choosing auth plugin

[eluser]NachoF[/eluser]
Im gonna be building an app in codeigniter soon.. Ill be using Datamapper (if it makes a difference).

My app requires two kinds of roles. admins and users.. no one can register, only admins can create users or other admins (and many other actions that users cant do, of course). Thats it... which auth plugins is most appropriate?... My main concern is that most auth plugins come with the ability for people to register to a site buti dont want that... Only admins well be creating users and other admins.
thanks in advance