There is nothing negative in saying that you cannot prove a negative. In fact not only can you not prove a negative, it is not necessary to do so, and the affirmation of that fact effectively rebuffs those who claim it is necessary to do so. For example, religionists, and sadly some agnostics, who claim atheists should prove that God doesn't exist, which is absurd, as would be an argument that those not believing in fairies must prove that they don't exist. In this regard, a statement that a negative cannot be proven is not a negative statement, but an affirmation of correct logic.