Well, they can give reasons. Of course, I can give reasons why I believe.
But the quesstion, “Prove that there is no God”, requires the proof of a negative, which is impossible.
What you have to do is to refute their arguments. Of course, with an atheist. upi are attacking their faith, and they don’t like it, and really cannot be reasoned with, IMO.
Let me restate. Atheists believe that there is no evidence for the existence of God. I disagree. However, just because no one has ever seen a unicorn, doesn’t mean that they don’t exist. But one can assume so as a working proposition. So if you believe in unicorns, or God, and you want to convince someone that doesn’t, you have to present your arguments and evidence.
This is not easy to do. But to me, a God that could be put in a test tube to see whether He changed from red to blue, like litmus paper, isn’t a God worth worshipping.
In the end, it is the Holy Spirit, not you or me, that will convince them.
What can they point to that shows there is no or cannot be any G-d? Just by saying "I believe there is no G-d" without any factual basis other than a statement doesn't seem like a valid argument. They're saying they don't believe therefor He doesn't exist.