Well that's fine if he doesn't mind losing his partition table!
dd if=/dev/zero of=/dev/hda bs=446 count=1
Would just remove the code in the MBR, not the partition records. If you have inadvertantly done this ken, you can use 'gpart' to scan your disk for partitions and rebuild the partition table.
I would also wipe out the first 512 bytes of any partitions (although not the first 512 bytes of a type '5' extended!) as this will remove bootloaders from them too (sometimes called PBR, or 'Partition Boot Record').
If you still can't remove it, it might be the result of a dying hard disk - I once had an old 850mb drive refuse to accept the DOS bootloader on the MBR, insisting on loading LILO instead (I was trying to re-use the disk in another machine) - even though the filesystem containing the kernel had long gone!