The file has to be .nomedia with a "." before it. Just create a text file on your pc, put it on the sd card, then rename it with your phones file explorer to have the "." in the beginning and remove ".txt" from the end. This does work, but that's part of the problem. Your phone will no longer be able to find the file to use it when needed. So for ringtones, they won't work. If it's a big problem for you to have these files show up in the media player, I'd also recommend just using a different player that only searches specified folders.