with your ipod connected, in itunes in the lower right hand corner there will be a button with an ipod on it.

click the ipod button, it will bring up the ipod options. deselect the auto update / auto synchronization (i don't remember the exact name of it, but i hope you get the point)

at this point you will be able to manage your songs on your ipod with out putting everything on.

ps to get rid of all the songs, on the left side of itunes select your ipod. this will bring up all the music files on your ipod. just select them all and delete.