Make a little box connected to a 5V switching mobile charger. Get some breakout USB ports. Connect the 5V rail
to a LED across a 220 ohm resistor, and in parallel connect the 5V and GND rails to the USB port. If a USB killer is connected to this port, that LED will flicker. The charger will probably die, though I doubt it will begin burning. I expect you will hear a little pop as the capacitor in the 5V charger goes bye-bye. It shouldn't trip your breaker box, though. Cost: less than 5 USD.
If the LED stays on, nice and stable, you don't have any problem. The USB killer works very poetically by charging capacitors, then driving a negative ~100 volts against the positive 5 volts of the USB port, bypassing protection circuitry. Less would probably be enough to murder a motherboard, but its creator is from eastern europe. In any case, this negative voltage would disrupt the light of the LED, and so serve as an indicator of attempted vandalism.
- 5V -> 220 ohms resistor -> anode of LED -> cathode of LED -> GND
- 5V -> 5V pin on USB port
- GND -> GND pin on USB port
Use a stripped USB cable, you only need the red and black wires.
On second thought, a properly rated schottky diode forward biased to the 5V pin of the USB, and one reverse biased to the USB GND pin might be useful in a PC/laptop setup. From looking at the page, it seems to present -200V DC to the 5V pin. I have a nice 0.18V schottky here, but only rated for 40V@1A. Most USB chargers and PSUs offer ~5.10V, and a nice schottky (>200V, Vdrop 0.1V) here can probably be used.