Physically destroying a disk with a hammer won't prevent the really clever bad guys from getting at the data. They are quite capable of reading data from fragments of a disk.
But - it's very expensive to do.
The same is true of a disk that has been erased using a properly-written disk erasing application. The data can still be read, but not by using an application. The really clever bad guys dismantle the drive and examine the surface using some very expensive kit.
BTW, be careful when physically destroying a modern disk. The platters are sometimes made of very thin glass, and you really don't want tiny fragments of that getting under your skin or into your lungs.
So its fine to use a good disk erasing application. The question then becomes "what is a good disk erasing application"? Well, its one that amongst other things, repeatedly overwrites each and every sector on the drive with various random data. I use Darik's Boot and Nuke, known as dban.
If you *really* want to be sure your data can never be read, then the only sure way is to physically destroy the disk by putting it into a physical hard disk shredder.
PPD