Remember that I'm playing the devil's advocate here. To me, Spanx's believer, there's abundant evidence. Now, I ask of you, atheist, how can you prove me that Spanx, whom you deny to exist, doesn't exist. How can you know for sure? Or do you not believe because you don't have sufficient evidence?
Huh? That's not at all what you were asking in your OP. You've totally changed your question, I suspect because your entire argument has fallen apart.
If you want to get into specifics, then 1)the people making the assertion have to provide the evidence. If they are ignorant of that, well, that's their problem and 2) why would I try to prove a negative to you? Do you realize how illogical that is? and 3) I don't believe anything without a reason to. Neither you nor any other theists has every provided any evidence or other reason to believe in <deity of choice>.
At this point, I'm not sure you even know what you are trying to argue.