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:
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'.
Too follow. The above does not work in 2.5