Typical towels do a pretty good job of not echoing ultrasound around too much, and also attenuate ultrasound going thru them. Ordinary sheetrock walls are really bad because they are great mirrors for ultrasound.
There are special materials for absorbing sounds at various frequencies, but hanging a bunch of large beach towels from the ceiling around each work area will likely be cheaper and just as effective. Two or three towels an inch or so apart should help a lot. You will still either have to put sound absorbent material on the ceiling, or make sure the towels extend to the ceiling.
The remaining path will then be by bouncing off the floor. That's not so simple. Some "area rugs" should help, but some of them reflect ultrasound much better than others. Ideally you want something that has roughness extending over ½ wavelength or so, but that will be hard to find. Beach towels on the floor will work to attenuate ultrasound, but won't last long as they aren't designed to take that sort of abuse.