That's moving the goalpost. The challenge was not to prove it to them, but to give an example of a provable thing, them claiming they would be easily able to prove the negative(or to prove that I cannot prove it? The wording is ambigous).

Asking me to prove something to someone in particular is a fool's errant. If you close your eyes, I cannot prove to you that you are able to see the sun.

But sure, if the perspective of bdangpublic is the measure of truth in this argument, then I claim that they exist instead. I should like to see them disprove their own existence to themselves. And simply arguing that I cannot be certain of their existence will not hold, since my perception is evidently irrelevant.