As rezag says...
Adding this line:
Options -Indexes
to the .htaccess file in the root directory of the site should turn off indexing for all directories of that site.
If you want to do it server wide (every site) then you need to add the same line into /etc/httpd/conf/httpd.conf
The section you need to look for should be something like this...
#
# Each directory to which Apache has access, can be configured with respect
# to which services and features are allowed and/or disabled in that
# directory (and its subdirectories).
#
# First, we configure the "default" to be a very restrictive set of
# permissions.
#
<Directory />
Options All
Options -Indexes
AllowOverride All
</Directory>
Otherwise the user could do it using the cPanel Index Manager (in the Advanced section) on each and every folder.