Removeable hard drive... take it when you don't want someone to use the PC. If they don't have a hard drive they don't have a computer.

http://www.addonics.com/products/com.../aechdsa35.asp

That'll definitely work.

Superglue the CMOS battery in the motherboard? That might take care of anyone trying to reset the BIOS password...

these are extreme measures however.