Noise cancelling headphones. Well, bodyphones. I mean, you want to make it so that the "noise" received from the outside enters and simply disappears. This means it is converted into local energy by being actively cancelled out by the opposite phase signal. This can be done by making a metamaterial that has a 'negative permittivity' - instead of being an insulator, which lets the signal through without reflecting or blocking it, the material actively cancels out the signal.
Spurious signals. Depending on your "metamaterial", there may be a delay before it starts actively cancelling the signal, which could be a blip on someone's very sensitive radar. But imagine you had a chameleon which concealed its own outline even as it displayed a blip on its body that zooms off to one side. Would your eyes follow the chameleon, or the blip? Similarly, you could try to confuse the radar by sending back "reflections" at times of your own choosing, to seem like you are further away and moving away fast. That said, quantum encryption is a thing now, and I imagine the military has ways of talking to itself to see if the photons it sent are 100% consistent with the coupled photons it examined. If they plot the percent consistency between their blip and the returns and there is a peak at a point where the reflection seemed weak but the consistency was higher, I imagine you're up for company.
Reflecting planes. The person who posted the stealth bomber can't be wrong - yes, directing the signal away from the radar seems like a good idea. Still, if you can have flying elephants I wonder if your adversaries have a bunch of Mickey Mouse satellites capable of looking for stray radar reflections when asked.