You may have success with some kind of pistonphone:
It's a device that provides a known sound pressure level (typically 114 dB SPL) that's used to calibrate measurement microphones. In order to achieve that SPL, the microphone has to be inserted into the pressure chamber of the pistonphone (opening at the top of the device, see image) where the microphone's membrane couples with the volume of air inside the chamber—the air is moved by a piston which creates the necessary pressure. Outside the chamber, no sound coming from the device is audible.
Now, you most likely would have to build something similar yourself. Since for your purpose you don't rely on anything being exact, you could try to just follow the key factors in play here: very close proximity of sound source (sound pressure source) and microphone membrane and a closed chamber. If you know exactly where the microphone sits in your machine and if you can access it, it shouldn't be too hard to build some kind of jammer. I assume that even something as simple as a cap housing a piezoelectronic speaker that fits on the microphone might give the results you are looking for. Basically the goal would be to distort every recording in a way that is beyond recoverable, like Rory Alsop mentioned in their answer (bullet point 1) without having to deal with speakers blasting a masking signal—that's mostly necessary when you don't know the location of a potential bug.