If your website isn't a community or otherwise needs to allow user registration, you should disallow user registration for security reasons. Here is how to:
- Log in to your Joomla admin panel
- In Joomla 2.x and Joomla 3.x, go to Users > User Manager and find the Options tab at the top of the page (In Joomla 1.5, go to Global Configuration > System
- Find the setting Allow User Registration
- Disallow user registration by changinig Allow User Registration to No
- Hit the Save button on the top right
You have now turned your Joomla website into a more secure place on the web.