Of course the easiest no-hardware-required / no-software-required solution to securing computers -- don't connect them to anything, especially power. This will reduce the possibility of malware, virus and hacker attack to nearly zero. As an added benefit, this cuts power consumption of the computer to zero as well. Granted, this does reduce the utility of said computers somewhat and is as useful as the solution given in the joke:
patient: Doc, my arm hurts whenever I raise it -- what should I do?
doctor: Don't raise your arm.