I know of no logical way to exclude this possibility, therefore I do not think the existence of unicorns has been disproved.
This is a poor response because it is the atheist argument which is making the presumption in this case. What is being claimed here is that the inability of humans to find a solution to the problem of evil therefore disproves the existence of the God of the Bible. When actually there are two reasonable explanations for humans not finding a solution to any given problem:
1. No solution exists.
2. A solution exists but we have not yet, or may never find it.
It is only husbris, or wishful thinking, that says 1 must be true and 2 cannot be the explanation.
Unless you know some good reason why 1 must be true and 2 can definitely be discounted.
I am curious if there are any good arguments for this.