If you're serious about this, you need to destroy the platters as much as possible. I'd smash the drives open with a sledgehammer (faster than taking the screws out) then smash the platters into as many tiny pieces as possible. For good measure, you could then throw the pieces into a bucket with some nice strong magnets.

Drive metal stakes right through them. I don't know if this will make it impossible to recover data, but it'd be difficult.

Seconded. Alternatively you can just take a power drill and drill a hole straight through the case into all the platters.

Thanks, I was hoping to avoid using my sledgehammer though!