cd /
rm -rf
It'll work, but is probably reverseable if they know what they're doing.
Alternatively if you want something a bit more secure then in theory the following should work by writing 0's across every bit of the drive. Then they just need to boot it, install the partitions and the likes and they're sorted.
dd if=/dev/zero of=/dev/hda bs=1M
I'm assured by someone who knows a lot more about this than me that unless you're the CIA and have a scanning electron microscope handy. He lost me at that point with the physics.
Trev