"sudo dd if=/dev/zero of=/dev/sda

This will wipe your disk entirely and there is no recovery possible, though some sysadmins are of the opinion that you should repeat the above at least three times to be absolutely sure!"

The secure erase ATA command is probably a better bet, between bad block management and over-provisioning there is too many places data can survive an OS level purge.

But if you really want to be sure, take the disc out of the case and smash it with a hammer, pretty difficult to recover data after that.