In order to prevent users from registering to your site the option must be removed. The following tutorial shows how to do this within the Joomla Administrator dashboard.
Removing User Registration in Joomla 3.1
- Log into your Joomla Control Panel.
- On the home page of the Joomla Control Panel, click on Global Configuration in the menu at left.
- Next, scroll down the menu at left and select Users Manager.
- Click on NO in order to disable new user registration or YES in order to enable it.
- Click on SAVE in order to save your selection.
Here you can see the option to Create an Account visible at the bottom of the login screen in the front end interface.
When the option is disabled, the option to login is available, but there is no option to create a new account.