Just because a dead hard drive will no longer boot an OS or the computer BIOS says its DOA, that does not mean the data is. There are utilities out there that will destroy hard drives (also called writing 1s & 0s to the disk). There are military standard DOD wipe programs as well.
And then there is the Verity Systems VS7000 manual hard drive destroyer which crushes the hard drive with the turn of a handle. The destroyer warps the hard drive platter so any data on it can never be read again. I've never seen anything like this!
Check out the other ways to destroy hard drive data