If you find one of your servers with the ugly directory listing enabled, there’s a quick way to disable it in Debian
# echo autoindex | a2dismod
# /etc/init.d/apache2 restart
For other Apache installations in other distro, you can simple find the Autoindex option in your config file and delete it manually, then restart Apache
EDIT: a cleaner and more elegant way to achieve the same is, as the comments section says
# a2dismod autoindex
thanks :)
The eternal fight between admins and computers 
a2dismod autoindex
this also work
Also -Indexes in the Directory or Location Directives in the vHost config o General config
a2dismod autoindex
apache2ctl graceful
Shorter and cleaner imho :>
Ok, a2dismod autoindex is better than the echo/pipe solution, you’re right :). But I do not agree on the apache2ctl, I always prefer the init.d script to reload conf/restart services, even if it’s a little more verbose