Add on domains? Config help needed?

I use a host which allows Add On Domains. I can use my webspace to host more than one website. Here's an example of what I am trying to do.

The first site I hosted on my webspace was called 123.com

When I add on another domain (456.com) this is created as a subfolder in the root directory of 123.com

Both work fine. The urls would direct you to the correct pages. What I want to ensure is that you cannot access pages with a url 123.com/456

As "456" is a subfolder of "123" it must appear to search engines to be BOTH two separate sites and that their is a folder called "456" in the site 123.com

How can I ensure that a Search Engine which is spidering my website cannot see the path 123.com/456?

I know a robots.txt will help to guide the robot, but I am looking for a cast iron way to ENSURE that 123.com/456 is not accessible.

Any help/suggestions appreciated.