Me__2001 was right about the magnet. You can get large industrial magnets used for testing welds. Wave that around your HDD a few times and it will scramble everything.
There is also a device used for bulk erasing VHS cassettes, it's basically a metal platform where you place about ten video cassettes, hit the button and it produces a strong magnetic field to erase everything. Or more accurately, rearrange the tape particles.
If you put a hard disk on one of those and switched it on for a minute or two, you'd be hard pressed to make anything out of all those 0's and 1's.
Deleting a single file and leaving an operating system intact would not guarantee permanent erasure.