Here are a few steps you can follow if you want to restrict access to Sitecore’s Interface on content delivery environments.
1) Open Internet Information Services (IIS) Manager
2) Expand your Content Delivery website
3) Click on the Sitecore folder
4) Double Click on Authentication in the middle content pane
5) In the Authentication Pane right click Anonymous Authentication and click Disable
6) Try visiting the Sitecore login page on the Content Delivery site and verify that you get an access denied exception
7) You might also want to copy the yourwebroot/sitecore/service folder into the webroot and update the paths in the web.config file for the various error pages.
Hope this helps.