You can also use RedirectMatch directive to deny access to a folder.
To deny access to a folder, you can use the following RedirectMatch in htaccess :
RedirectMatch 403 ^/folder/?$
This will forbid an external access to /folder/ eg : http://example.com/folder/ will return a 403 forbidden error.
To deny access to everything inside the folder, You can use this :
RedirectMatch 403 ^/folder/.*$
This will block access to the entire folder eg : http://example.com/folder/anyURI will return a 403 error response to client.