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The printer itself, and this is quite common with many makes and models, cannot have both its network interface and its USB interface active at the same time.
The reason for this is simple: if the printer had two interfaces active simultaneously, then it would be subject to getting two print jobs at the same time from different sources. This could require additional circuitry, or else would mix up two different print jobs at once in the printer's memory. Using one interface at once, simplifies this sort of problem out of printing operations altogether.
The way to disable the wired (USB) interface is simple: unplug it from the USB cable.