Recently I have deactivated the user registration on few of my Joomla websites, because I realized that many users are registered with fake emails or details. I was surprised because there were not registration form or link displayed on the respective website. I'm confident about automatic registrations, that are made by different bots and software, so I decide to write a quick tutorial on this topic, for those who are having trouble disabling the user registration on their Joomla 1.7 website.

Why should you let invalid users or bots to register on your site and using your server resources and store useless info in the database. Here is how you can proceed:

Disable User Registration in Joomla 1.7

1. Login to the Administration panel
2. Go to Users > User Management
3. Click on Options button at the top right hand
4. Disable the registration and save the process
Don't let automated software to use your site.
If you really need this feature, I highly suggest you to add an anti-spam plugin to your site. This will minimize the unwanted registrations. Good luck!

source : http://joomlathemes.co/tutorials/50-how-to-disable-user-registration-joomla-17