Again, let me reiterate that caveman clubbing really is overkill.
Especially if it's a good, working HD that can be re-used.
That said, if you really insist, I'd unscrew the top cover of the hard drive, and smash the platters itself. I'm sure enough pounding with a sledge hammer will also do the trick, but opening it up gives you clear visual confirmation that the platters are properly "smashed".