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:
If you do not want visitors to your website to be able to register themselves as your Joomla! website users, do this:
For Joomla 1.6+ you must go to User Manager from the User menu. Then click on the options box and turn the parameter for Allow User Registration to 'Off'.