Is it possible to jam the transponder signal without actually turning off the equipment?
Generally, no. You do not jam the transmitter - you jam the receiver. But the receiver is on an Iridium satellite. If you have to jam from the transmitter's position, you have to use 100 times as much power, and an antenna that is just as good - and even that may be not enough, depending on the modulation of the signal.
Maybe not the transponder but that could be turned off by a breaker behind a panel in the cockpit. It would require expert knowledge but not difficult.