This tutorial assumes you are already logged in to your admin control panel.
1) Click Site.
2) Then Global Configuration.
3) Go to the System settings.
4) Next to Allow User Registration, click No.
Users will now be unable to register a new account for your Joomla site.
At this point, you can also choose the default group that will be applied to new users upon registration and set whether users have to activate their new account through email before they can log in.
5) Click Save.
That's it! You now know how to disable user registration in Joomla.