Rocbkbox Utility is absolutely useless for uninstalling Rockbox on the iPod Classic 6G.
The only thing it can do is delete the /.rockbox directory, you could do that yourself, but that still leaves you with emCORE (which isn't supported, installed, or removed, by any Rockbox product).
Depending on what method you used to install emCORE on your device, the device drivers are probably in what I affectionately like to refer to as "a fucky state". Plug the device and uninstall any and all drivers associated with it from the Device Manager (this assumes you're using Windows, but that is a probable guess I would think). Then completely remove and re-install iTunes (
iTunes 9.2.1 recommended).
Force the iPod into DFU mode (the same way you did to install emCORE, by holding menu and select with the device plugged for 10~15 seconds), and then restore the device with iTunes (which should prompt to restore the device upon launch).
[Saint]