the easy way is to put a blank index.htm and index.html and for some sites main.htm and main.html, or put all four. When someone goes to that folder, all they will see is a blank page, unless they have access to the actual file in the folder itself.
for instance, if I send you to a folder with pictures of my backyard, you will see nothing other than some warning page.
http://glenburniemd.net/backyard/
If instead I send you direct to a picture in it;
http://glenburniemd.net/backyard/garden.jpg

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