How to disable all joomla register an login, and using JFBC only

The best way to do what you're looking for is to simply override the standard "Login" page of Joomla. By doing that you can remove the login form and add the Login with Facebook button and customized things however you want.

To create an override, simply copy /components/com_users/views/login/tmpl/default_login.php to /templates/<YOUR_TEMPLATE>/html/com_users/login/default_login.php (Create any of those directories as necessary).

Once there, edit away. You can basically remove almost all the code, since it's almost all for the Login form. Then, just replace it with your own text and the {JFBCLogin} tag (for the Login with Facebook button) and you should be good to go.

I hope that all makes sense, but if you have any questions, just let me know.

Thanks,
Alex