In Older versions this option was at Global Configuration section. At joomla 3.x seems that this option has been moved.
I want only my existing users to be able to login.
Has anyone any idea where this option is available? Thanks in advance!
Under the Component tab, find the following setting:
Allow User Registration
If set to Yes, new Users allowed to self-register.
To disable user registration, set Allow User Registration to No, and then click the Save button in the top left of the page.
After disabling the registration option, you will notice that the Create an account link within your Login Form will no longer show, as users can no longer create an account on their own.