Is it possible to prevent Magic Keyboards from pairing without disabling Bluetooth entirely? In our office, we have a whole bunch of Magic Keyboards, and for various reasons too complex to go into here we have a habit of moving them around and connecting them to different Macbooks. As soon as they are connecting with a Lightning cable, they will typically auto-pair with the machine via Bluetooth, meaning that even once the cable is removed, they continue to be connected. It's both annoying (and potentially even a minor security risk) for someone else to have access via a keyboard they just pick up.

We can't return the Magic Keyboards; is there a way to either disable their Bluetooth behaviour or prevent the machine from auto-pairing, so they only work over USB?
No. (Not that we’ve found)

These devices are designed to pair like rabbits. Rapidly, without delay and as often as possible. 

People prank each other at our work as well by taking a keyboard and plugging it into all the iMac down the row. (Or shuffling up the mice)

The best you can do is set a name for each keyboard and laugh when a charge turns into a pairing event.