How to remove a song from Google Play Music without removing it from the storage?
Hello everyone and thank you for your answers, I've finally managed to accomplish this with a very simple solution: The Android media storage service will not index media files inside directories which are hidden (preceded by a "."), but also those which contain a file named ".nomedia", so I just created on all the directories I wanted to exclude and that was it. Hope it helps somebody else. 

P.S.

I didn't want to hide the entire directories since they're my phones ringtones and notifications directories and I still want them to be visible to the sound manager to use them, I just don't want them to be seen by Google Play Music.

I found the solution here: http://www.guidingtech.com/15563/hide-certain-files-android-music-player-photo-gallery/