You can't prove the non-existence of anything.
It's naive to think that definitive disproof of God is required to make atheism reasonable. Consider two factors which, while meaningless in and of themselves, together form a reason to disbelieve in God.
First: lack of evidence: The lack of empirical evidence for God doesn't mean much by itself. There is also a lack of empirical evidence for the notion that one of my fifth century ancestors liked to play games of chance. But since we know that such things are possible, it would be foolish to dismiss this notion out of hand.
Second: lack of explanation: The lack of an explanation for how God could exist doesn't mean much by itself. Before anyone had a clue what nuclear fusion was, there was no explanation for the fact that the sun was still shining after untold millions of years. And yet, we wake up to the light of empirical evidence that the sun has been shining to this day.
First AND Second: Though they don't mean much by themselves, taken together, the latter considerations mean a great deal. There is no empirical evidence for the existence of God AND no explanation for how a limitless field could have a limitless number of effects and behave in a manner consistent with arbitrarily high intelligence. AFAICT, these considerations leave the rationalist with no good reason to believe in God.
A militant agnostic could stick to the claim that the latter argument warrants only a suspension of judgment. However, no one could actually remain so intransigent for the purposes of any real-world inquiry. Did Mr. X murder Ms. Y? Well, there's no evidence that Mr. X did the deed, and no explanation of how Mr. X could have killed Mr. Y, since Mr. X has been in a coma for the last five years, up to and including the time of the murder.
Suspension of judgment is not a rational response in this instance. Neither is it rational to suspend judgment on whether non-believers are damned for all time in the face of a) no evidence for the claim, and b) no explanation of how this claim could be true.
I think that the OP might be unaware of the distinction between strong atheism and weak atheism. Strong atheism is the claim that the evidence against the existence of God is good enough to rule out theism. Weak atheism is the claim that there is no good reason to believe in God. I've posted on religious forums for years, and have met many weak atheists. I'm sure I've met one or two strong atheists, but I can't remember who or when.