Beware that you will only hide the directories on the same level. If you link to any object on any other level, the user doesn't have to guess directories any more, and he can browse at least a subtree. Put empty index.html-files (or even perhaps with redirects) everywhere.

If you're on an apache server, and it's configured to read .htaccess files, you can use it to turn off browsing for an entire subtree (if I remember correctly). However, what you can do with an .htacces-file depends on the server setup, so you have to ask the server administrators. Read all about .htaccess in this tutorial.