They don't have to give back any real money in my opinion. If they just give back the zen used, no one will care about AD. Levels = Time; people don't want to feel it wasted. Zen = Real Money; take away items bought with zen, give back zen. No harm, no foul.
I have a Crazy Idea! Reset everything except Level (and maybe even non-exploitable items), then reimburse all Zen purchased since "open beta" launch. I know, crazy idea.