Select the Users menu option. Now down to User Manager and click.
Now let’s scroll right. Next click on the Options button.
Here we can change settings relating to user registrations.
Click No for the Allow User Registration field.
Select the New User Registration Group dropdown list and select a user group as the default registered user group.
We will leave our registration group as Registered. With the New User Account Activation set as Self, users will receive an activation email to complete their registration process. You can optionally choose to be notified by changing the Notification Mail to Administrators to Yes.
After choosing all of the user registration settings, click the Save & Close button.
Let’s see how our changes will affect the frontend of our web site. Click on the View Site link. Now scroll down to view the change in the Login Form on the left. Notice the Create an Account link has been disabled under the Log in button.
You now know how to prevent users from registering in Joomla