Lump hammer is fun effective
I found that a hard drive is surprisingly tough. The circuit board shattered with the first blow, but the casing never broke open despite smacking it very hard.
In reality, sourcing a replacement circuit board of the correct PCB revision that will actually allow data to be read would defeat most people. You'd need to buy a LOT of eBay drives for spares. And the shock would probably have ruined the internals anyway.
The commercial way to do it is either to get a big hard disk crusher (I've sene one in action at the InfoSecurity trade fair) or a hand-operated device which drives a big pin right through and out of the other side, bending any metal disks and shattering any glass ones.
MI6, so I'm told, require the oxide layer to be ground off the platter and returned to them in a little bag.