Disabling user registration
From Joomla! Documentation
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This tutorial is for Joomla! CMS Version(s) 
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 1.6+[edit]
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'.
User registrations are enabled in a default Joomla! installation.