While no doubt somebody will post the 'official way' (ie. altering the httpd.conf file) a rather simplisitic option that may interest you in the mean time is simply placing an index.html file in the directorie(s) concerned.
Apache will only resort to file display listings when it can't find any html, cgi or php file to chew on. If you provide it an 'index.html' then apache will always opt for displaying it rather than your file listing.
Regards,
Luke