Dan Smith's answer is most likely the correct answer: it cannot be done.
However, entropy does not always need to increase. Entropy is permitted to remain the same. Such systems are called reversible systems.
Current quantum theory suggests that everything is done through reversible operations. However, to reverse the operation, you do need all of the outputs from it. Unfortunately, electromagnetic radiation is one of the potential outputs from such a system and it tends to propagate outward at the speed of light. In the real world, this is typically assumed to be an irreversible process, but with your handwavium, we could reflect this energy back. We could actually start to talk about reversing macroscopic effects!
The first step to this process would be to define what "human" is. This is a known challenge in philosophy with no agreed upon answer, so I don't expect to find one soon. However, if you did find a definition that was acceptable and could be expressed as some equation of space and time, you could take the Fourier transform of that expression to get the frequency distribution of "human." With this, you could construct your box with its perfect reflective walls into a structure which is resonant at those frequencies. Any human energy which is put into this system would become a standing wave within it, capable of existing forever.
Of course, any one of those steps would be considered an insurmountable hurdle, but the current scientific theory doesn't forbid it. The book Manifold Time by Stephen Baxter plays around with it at one point.
And no, I do not have any idea what the frequencies of "human" are. Find the meaning of life, and you'll probably be able to derive the frequencies from that!