That seems like a pointless line of argument -- in the sense that failure to "prove" there was nothing before the big bang does not help advance an argument about whether a particular "thing" or "prior existence" (or God or whatever) might have existed pre-bang or still exists. If the tools and theories at our disposal do not allow us to "peer past" the big bang, then we have nothing to discuss about what might or might not have "existed" before that point in time.
Now, a person believing in an inerrant Genesis might say, "But, I do have proof of what existed before the big bang -- God existed and he lit the fuse that ignited the big bang -- it says so right there in Genesis (fiat lux) and that is the inspired Word of God. QED."
Leaving aside Genesis' alternative creation stories, shuffled together at some point (perhaps during the Babylonian exile), is this proof of anything? Not really -- some people claiming they were told something they didn't personally witness (or were inspired to write down something they didn't personally witness). And we can't drag God into a courtroom and cross-examine him/her/it, and we can't cross-examine those who wrote down Genesis, so that particular writing about God and his/her/its existence and actions cannot be conclusive evidence of anything (other than that one or more forms of Genesis were written down by someone at some point). Not only is absence of evidence not evidence of absence (as someone noted earlier), it isn't evidence of anything affirmative either.
I could say that I have it on good authority that our universe (big bang and all) was pooped out by a giant space turtle in a preexisting universe. You might mock my assertion and my lame appeal to ambiguous authority (who said it?, Space aliens told me, where's the proof? left it in my other pants, where did the prior universe come from? an earlier space turtle, and before you ask, it was turtles all the way back), but you can't "disprove" my assertion -- you can only choose not to believe it (or to assert that I'm lying and that I don't even believe it). To which I can only reply, OK -- your choice. I don't have to prove it and you can't prove I'm wrong.