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.