Obviusly they do not consider you a valuable customer, they imply that the relationship is not mutually beneficial anymore an skewed in your favor.
As for the proof, it seems like the want you to send proof that you actually paid for the miles you received (which of course you can't do because you returned the tickets and never had to spend the $100K to get the miles.)
Asking them to withdraw those miles at this point is not a solution they are willing to consider. They feel like you wrongly earned some miles and they do not wish to transact any further business with you.
It's time to move on. Join different CC and FF plans and start from 0. It looks like AA isn't going to budge on this and it looks like they are very comfortable with their decision. It'll probably be a waste of your time and their time to pursue this further.