Erase partition without recreating one?

I've got an external USB hdd that I wish to use entirely as a TrueCrypt volume. There was one pre-formatted partition, so I'm trying to erase that with Disk Utility. But no matter what I do or try, whenever I Erase the partition (or the entire disk), it keeps automatically re-creating a new partition. How do I just erase the partition, without recreating one, thus leaving the entire disk empty (i.e. without even one partition) ?