The least violent way would be to wipe it with a magnetic wiper. Seagate's website actually gives the gauss power needed to wipe drives to different levels, from wiping the disc platters right up to demagnetising the motor windings. You would need special wiping equipment for this.
There's also a device that clamps the drive, and then you wind a geared handle that bends the drive into L-shape.
Without special gear, I'd use some of the many multiple-pass wipers, then do a low level wipe to delete the firmware stored on the drives. Then I'd remove/break the PCB (many drives need these to be matched individually nowadays), take off the stickers over the air expansion holes, and hit it a few times with a hammer.
If you want to go all the way and you have a torx screwdriver, just take the case off and break the platters and heads.
A hot fire or blowtorch would probably work due to the heat breaking down the magnetic surface and demagnetising the heads and motors. That would probably be illegal in the EU as it wouldn't be proper disposal of electronics and would most likely make some kind of poisonous smoke for you to breathe.
Ridiculous...
