To disable Joomla 1.7 User Registration go to:
Users -> User Manager
Click the Options button in top toolbar.
This opens the User Configuration settings.
In the Component tab set "Allow User Registration" to No.
Click "Save" button at top.
This will remove the "Create an account" link from the Login page and Login Module.
This also blocks direct access to index.php?option=com_users&view=registration
