Is it possible to prevent Magic Keyboards from pairing without disabling Bluetooth entirely?
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.