I would stop by a local body shop and ask. If they are good people, they can give you a local place that can do it if they can't. Polished aluminum generally has a clear coat on top to prevent tarnishing. I am guessing the dealer thought, "hey, it's aluminum, we can just buff that out." Unfortunately, that is not the case.
Back to the dealer. If you asked them to try and they warned you it could get worse, that is one thing. If they did it on their own without you knowledge, then you need to have a discussion with the service manager.