Open it up with a screwdriver. Take the whole thing apart. Now, inside are beautiful, polished, mirror reflective discs. They arecalled platters, and they store information. Anyway, take those out of the hard drive, and smash them with a hammer. It's not so much as smashing the case, you want to destroy the actual things holding the data, and well, the case and screws don't hold the data. So, you could smash it 10 times with a hammer, but if the platters are still intact, data can be pulled from those platters.
So just smash the platters apart, no more data.