How to disable user registration in Joomla 2.5

Step 1: Log into your Joomla Dashboard / Administrator backbend

Step 2: Click the User Manager icon

Step 3: Click the Options icon in the top right

Step 4: Under the Component tab, set Allow User Registration to No and then click Save & Close

Step 5: You have now disabled user registration! Your Joomla 2.5 website will no longer have the new user registration option.

You also can refer here.

http://www.cmsmind.com/joomla-2-5-tutorial-how-to-hide-user-registration-form/