Take the appropriate screw drivers and dismantle the drive. Take the ring lose that is holding the platters and remove the platters. take them individually, and rub the surface over and over with a strong magnet, (the one in the hard drive is perfect for this) going in random strokes every which way, doing this to both sides.
Then take some very coase steel wool or a wire brush, or even a screwdriver, and scrub both surfaces until they are extremely scratched up.
The disks will be made of either metal or glass. Take that hammer as someone has suggested, and smack the platter smartly in the middle, on the hole. If it is made of glass it will shatter (do this in the bottom of a garbage wait for easy disposal).
If it is metal, try to use the hammer to flatten it into a cube or ball, sometimes they are too tough, so just lay the flat and smack them many times, but turn the hammer slightly so the edge of the hammer impacts the disk creating creases and smashed areas.
Discard one disk each week in your garbage, or go around town and discard them into dumpsters. just throw them away at different times and locations.
If you have a program called eraser, before you do all this run a DOD erase on the entire drive and be sure to wipe the cluster tips too.
