You can delete the playlists completely on your computer, I am not sure if when you sync your iPod it'll also remove them from your iPod.
If you don't wish to delete, select the playlist of your choice (Recently Added or Recently Played) and then click File ---> Edit Smart Playlist to change the playlist's settings... This way you can, for example, have iTunes remove a recently added song from the Recently Added playlist after 1 hour. _________________
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