I want that the window does not display the create user

I want to only display the coorect fields for the facebook user to indicate the good user and email already created in Joomla.
I try many things but I cannot correct that.
I do not permit the creation of new user in Joomla user parameter but the screen show the two (user and password) and the new create user account.
I want to ONLY join the facebook user with already created user by me (Administrator)
Can I do that and how?
I use Joomla 2.5.9 and K2 2.5.6

I try to change the parameters in K2 and in jfbconnect. I try also with sclogin and now I use K2 user for login but I cannot remove the create user fields.