In order to try to refute this proof you have to deny objective morality, which is absurd. Watch this 3 minute video to see the absurdity of moral relativism which attempts to deny objective morality. Before commenting please take the time to watch the video, and in your comment please indicate that you've watched it, and answer this question: Yes or no - is it objectively wrong to abuse babies for fun? (**Any comments that are trying to refute the proof and don't answer this question will not be published)
Another point relating to objective morality is that everyone lives as if objective morality exists, so when professed atheists deny it with their lips, they are contradicting that with their lives - every time a professed atheist accuses a Christian of dishonesty or something else they think we SHOULD not do, they are borrowing from the Christian worldview and confirming that in their heart of hearts they DO know God.
Further Reading / Research:
Atheism - no objective morality - creation.com.
Is-ought problem - Wikipedia.