I'm reminded of the exasperated Bertrand Russell faced with the young Wittgenstein: "He thinks that nothing empirical is knowable. I asked him to admit that there was not a rhinoceros in the room, but he wouldn't. I looked under all the desks without finding one but Wittgenstein remained unconvinced." It is Wittgenstein here who is being obtuse and in the grip of a silly theory. Of course we can establish empirical propositions both positive and negative – for example, that there are five desks in the room and no rhinoceroses.
By any sane standard, it is just plain false that you can't prove a negative, and that supposed "discussion-killer" should itself be promptly killed off.
I've discussed this before myself; and even a very basic amount of research will uncover the fact that philosophers have been pointing out for at least two hundred years now that the adage is not really true. Of course, there are contexts where you can't prove a negative to a desired standard or level of proof. But it's hardly a general truth.