The Index Manager in cPanel may do what you want as long as you don't have a page named "index.xxx" in the folder. I haven't used the feature, but what it may do is to add a line to the .htaccess file in that folder.

"The index manager will allow you to turn indexes on or off for a directory. If indexes are off, people will not be able to see what files are in a directory that lacks an index file."