I tried to delete downloaded songs turning off button "Download" inside one of my playlists, these are the bad results:
1) The green symbol of "downloaded" disappear from every song of the Play List;
2) It's not possible anymore to play the deleted songs in offline mode, so it seems the Spotify really deleted the downloaded songs;
3) PROBLEM: using my file browser, I see that the files have been removed from my SD card... So, I cannot play songs in offline mode, but the SD card is quite full.
The only working solution is to delete all Spotify data, but in this I delete all downloaded songs and that's not good.
Any idea? Spotify manages downloaded material in a very bad way... I don't if I will renew my Premium account.