Although FORMAT might take a long while to complete its task, it takes only a couple of seconds for it to wipe out the root directory and file allocation table. You could hit Ctrl-Break after a short while and the disk would be unreadable.
Another possibility is to run FDISK and delete the primary partition. At the next boot-up the BIOS would not see any bootable partition on the disk and would try to boot from drive A: instead.
