There are two points here - what works, and what you should (or should not) do.
When I'm done with an old HDD, I open the top and heat it red-hot internally with a small DIY gas torch. It takes a few seconds from start to end. No magnetic data is going to survive the heat rise, which destroys/randomises the magnetic domains with absolute certainty, even if the plating on the platters wasn't oxidised/charred/burnt off and peeling. The case is easy to open too.
Notice the emphasis above: it's what I do. Almost certainly it isn't what you should do as a business. Nor is drilling, acid, electrocution, thermite, or any other fun activity. There are serious issues to consider before letting staff loose on the disks.
As an individual I'm fine doing what I prefer. As an employer your company is probably legally liable for staff safety and any accidents (in most if not all countries). I wouldn't allow my staff to do what I do personally. All it takes is one accident with a drill, due to exuberance or carelessness, some metal swarf to hit an eye, or anything else, and you can expect a visit from the lawyers who will ask you exactly what training and control your company gives, when it turns ugly.
Most of the alternatives suggested in other answers are a lot of fun - until they go wrong. At which point one person is in the line of fire. You.
Alternatives - top off case (ensures exposure as other answers state), and ideally some action that physically damages the platters (in any manner) but doesn't incite reckless conduct or risk an accident. Perhaps buy a hand-held demagnetiser (mains powered, produces a powerful local magnetic field designed to randomise data, has little or no harmful potential). Less exciting but a lot safer.