Funny that. I seem to recall one or two posters some years ago jumping on a band wagon specifically stating that you could not prove a negative. Of course, it was a negative that they didn't agree with. You were on the forums then, do you happen to remember who they were by any chance?
Anyways, another poster recommended a book on the forum, again, a few years ago, that I went out to find and finally bought. It is entitled Historians' Fallacies: Toward a Logic of Historical Thought by David Hackett Fischer, who is a pretty good historian himself. On page 47 he names 'The Fallacy of the Negative Proof' which he states 'is an attempt to sustain a factual proposition merely by negative evidence.' Further on, on page 62 regarding historical evidence, Fischer states that 'evidence must always be affirmative. Negative evidence is a contradiction in terms-it is no evidence at all. The nonexistence of an object is established not by nonexistent evidence but by affirmative evidence of the fact that it did not, or could not exist.'
This subject also came up in my military history masters' program, and academic historians tend to agree with the tenet. Quite simply, you cannot prove a negative. That tenet is part and parcel of historical investigation and argument.
I would highly recommend this book to anyone interested in how to present historical evidence in attempts to prove or argue about an historical subject.