permanently destroyed by using the following command (the drive must be unmounted first, which means that a LiveDVD or LiveUSB is useful in this case (warning - this will take a very long time):
dd bs=4k conv=notrunc if=/dev/zero of=/dev/sdX
Confirmation of whether or not any non-zero bytes exist:
dd if=/dev/sda | hexdump -C | grep [^00]
encrypting them before deleting them in the usual way.