Read this:
http://en.wikipedia.org/wiki/Evidence_of_absence
Proving negatives actually brings its own philosophical debate.
Really it all depends on what you mean by proof. If we are saying that evidence gives a high level of certainty then we might fairly comment that something can be proved. It is rather unnecessary to substitute in 'I believe the evidence available shows that something is most likely to be true taking into considerations the limitations of our abilities to make the correct observations' when you can simply say 'there is proof'. On the other hand, I think that the only science that has any proof in the strict sense of the word is maths.
In the context of this discussion, where there are obvious and overwhelming limitations on our ability to make what we might perceive to be the correct observations, proof is a word that is best avoided. All parties are better off sticking to 'merely' weighing the evidence.

