Celestina's suggestion is pretty much the only way you can reverse it publicly, by returning the money.
But if you're dealing with someone that has gotten famous for pedophilia and is not in jail then I'd suggest a secondary option: Do nothing about the art they got from you, don't bring attention to it like this, ask AB to delete this thread and their twitter post, delete your public uploads of it, delete your public posts about it, and never work with that person again.
Not shining light on it is the best way to let things disappear on the internet, let that person get bored of that art naturally and stop using it, because when they find out this post they (out of spite), and everyone else (out of morbid curiosity), will be much more inclined to look for that art and keep using it. It's how the internet works. But yeah, though choice to make it a public and legal battle, or let it die quietly and when anyone asks just tell them you only were told about who that was after you worked with them.