What I'm going to do is wait seven days before I toggle the online off-line toggle and that should wipe out all my saved playlists and that should theoretically delete that one playlist that I can't UNdownload
For becoming a Spotify Premium user you need a credit card number and personally i have never had any problems when cancelling it.
They playlists you create or you follow will not destroy, doesnt matter if you are premium or free.