Deny folder and all the subfolders in Apache with .Htaccess

You have a folder inside which there are library files which you don't want to show it to the end-users.
RewriteRule ^subfolder/ - [R=404,L,NC]
You can send any type of HTTP status code with this. If you want to show a Forbidden error you can show R=403. L tells that this is the last rule and discard any rule after this. NC is used for Case Insensitive.