Disabling user registration
From Joomla! Documentation
This tutorial is for Joomla! CMS Version(s) 
User registrations are enabled in a default Joomla! installation. However there might be cases where you use extensions as a replacement and wish to turn off the default Joomla user management. To do this you must perform the following steps:
Joomla 1.5[edit]
If you do not want visitors to your website to be able to register themselves as your Joomla! website users, do this:
- disable user registration by changing the Allow User Registration field in Global Configuration to No. To learn how to do this see Changing user registration settings.
Joomla 2.5[edit]
- Navigate to Users → User Manager then click on User Manager. Once in User Manager Screen, click on 'Options' Toolbar button to show the pop up window, as in image above. Click to fill bubble next to Allow user registration, then click save.