Index of Files in Directories without index.html


I tried searching for this but can’t find anything on it. I have a few directories on my site that I use to put photos on message boards and whatnot. Since I don’t have an index.html file in these folders, users can see the URL and go and look at all the files in the folders. I know there is a way to change that so they don’t see the list of files - how? Any help is appreciated. Thanks.