Thanks for the correction. I am woefully ignorant in electrical engineering.
How about a dust? If you could manufacture tiny corner reflectors the size of dust grains and suspend them in the air in front of the gun, they would deflect and/or absorb the microwave energy in the beam...
Corner reflectors work pretty much at all frequencies down to those whose wavelength is comparable to the size of the reflector. Thus you don't need to make them that small. Ferrite dust might make a good absorber, but so would a water spray, the water spray would also reflect pretty well at those frequencies.