[jacobwcarlson]

Not if you access to the supply chain for the construction of the computer.

[retbull]

Or if you simply do what the REDACTED government did when they released StuxNet which is infect thumb drives and leave them near by the building in the parking lot. People pick up and plug in anything.

[mirimir]

Ideally, people don't use random USB drives in air-gapped computers ;)

[stephengillie]

Or you use one of a number of methods to disable unnecessary USB ports, so there's nowhere to plug anything in.

[mirimir]

Or you miswire them, disconnecting data lines from the ports, and routing bus power to the data terminals on them ;)