You can flood the zone with inaudible white noise.
It’s a White thing, you wouldn’t understand.
Practically speaking, since audio in that range is fairly directional, and you need a lot of power, the utility of such a jammer is questionable. It would of course probably drive away every rodent and dog in the neighborhood.
Some people suspect the sonic injuries that occurred in Cuba to our diplomats resulted from interaction between ultrasonic sources, so I would not encourage anyone to make such a jamming device at home, or expose anyone to such a strong ultrasonic audio field. It could be hazardous, so leave that kind of thing to research labs.
Remove the magnetron from a microwave oven.... turn it on....
Lots of white noise
Unsheilded ignition type system..
You just use a small RF white noise generator. Mics are passive so pretty much impossible to detect (think phone). In the old days you had to modulate tape to record so you could detect that, not true now. So you have to “wash out” the audible content. Not foolproof because you can extract the content with filtering.
I don’t know about “inaudible” white noise but I have used white noise generators to adjust speakers before... many amplifiers have the function built in so you can use a calibrated mic or spectrum analyzer to adjust for speaker weaknesses or dead spots in your listening room.
If you are truly worried about being recorded simply make sure there are no recording devices allowed where you are meeting... cell phones should be put in a metal box at least xx feet away so BT cannot connect and the phone itself cannot hear... play white noise over the stereo... if there are windows dampen them out with sponge rubber or some other deadening.