I have this same problem. It seems that Spotify saves its offline playlists to the on board memory and not to the memory card. Once it has saves a playlist locally it appears that you cannot delete the local content!
I have the same problem with a BlackBerry Curve 3G. I can't delete all my apps for free the required space. Currently I've only 25mb free now on my phone battery, although I have lot more free space on the external memory (more than 1GB).
How can I manually delete the Spotify playlist for free the required space?