How is your iPod set to get its music? Does it automatically update, update selected playlists or manually update. For the first two, delete those playlists from iTunes and they'll automatically disappear next time you plug the iPod in. For the last option (manually manage songs), delete them from underneath the iPod when it's plugged into iTunes. They'll appear alongside your other iPod playlists on the left hand side of iTunes. 🙂