The easiest way to stop the directory indexing is to put a blank index.html file in the directory. This will pull up the blank page every time someone tries to access that directory in a browser.
Another option would be to create a .htaccess file in that directory. Create a text file named .htaccess containing the following line:
Options SymLinksIfOwnerMatch -Indexes
Now upload this file to the base directory for which you wish to disable directory indexing. This directory and all of its subdirectories will now have indexing disabled. This Apache directive will disable directory indexing (the contents of a directory being displayed in your browser) having the same effect as displaying a blank index.html page.