The "Terabyte" thing is something we are aware of - it's an odd little cosmetic error.
However, desyncing tracks may work a little differently. First ensure that you have the latest version of the iOS application - Check over at the App Store on your device and see if there are any updates waiting for you.
Then, ensure that your Auto Lock is turned off for a moment - You might need to have Spotify active for a few moments. Head into Spotify and then the playlist you don't wish to have offline anymore. Turn off the "Available Offline" setting at the top of the playlist, then leave the application active for the next 5-10 minutes on your screen. This may remove these tracks from your cache. Failing that, you can reinstall the application, which would remove these tracks as well.
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Spotify