To disable directory listings for a website using apache2 and ubuntu, edit the file
/etc/apache2/sites-available/default
Change the lines:
<Directory /var/www/>
Options Indexes FollowSymLinks MultiViews
to put a - in front of Indexes
<Directory /var/www/>
Options -Indexes FollowSymLinks MultiViews
save the file and then restart apache using
/etc/init.d/apache2 restart