well short of taking sandpaper to the disk platters...
you need a 5 swipe disk scrubbing tool.
will do nicely and is used by corperations and governmental departments alike, (as you can read in their portfolio) It's also free.
Data can be recovered in a number of ways.
1: you only delete the file
problem, only the poniter to the file is erased, the data remains
2: you format the disk
problem, only the first sectors are eraed, data remains
3: Low level format,
problem, whilst the entire disk is re-written there is magnetic residue that exists on the disk surface, this can, (when analised) enable files to bereconstructed).
4: disk scrubbing
the disk is written ll bits to zero
problem : magnetic residue remains
5: disk drilling
driling a hole in the disk
problem, 99% of the disk is still intact, and datastill exists on the platters which are mostly undamged
6: Dunking in water,
problem: disks are sealed so thisdoesn't damage the disks
7: smashing controller board
problem : new control boards can be fitted.
8: five pass disk scrubbing
all bits are written to zero, then all bits are written to one, then zero, then one, then zero, then one, then zero then one, then zero, then one
problem, it take so damn long!!! but data is irrecoverable.
what do you have that you are worried about the police finding?