Permanently destroying a drive is easy. Since I build PCs on the side, I often end up with old junk which I part out , save for the hard disks which I destroy for the clients.
A ) remove and discard the circuitboard. A drive's geometry is almost impossible to read without an exact match. If its an old drive the odds are its good as dead. However if one is paranoid, or the data is that sensitive--
B ) Dismantle remaining drive assembly, remove platters and destroy with tin-snips. Discard the bits and pieces in random public trashcans, such as the ones outside of McDonald's, Supermarkets, etc etc. If all of the puzzle pieces aren't in one place, the odds are quite good nobody will ever be able to do anything with the remains. This is the same technique I use with old credit cards.. I chop them up, and dispose of the bits and pieces in different locations.