I was recently bombarded by someone who voted up every one of my questions and answers. That caused me to be awarded the mortarboard badge, but when the votes were reversed the badge remained and remains to this day. I do not wish to ever be awarded any badge to which I am undeserving, so how can I return that badge?
You don't, and no one expects you to.
Basically, SE has made it their policy that they won't revoke badges awarded even if you no longer meet the criteria simply because it's not worth people complaining about the reverse of what you're asking about ("where'd ma badger geaux?"). I think there's an exception for tag badges that get deleted, but that's about it.
Enjoy your shiny new badge.
I will say that the day that you got this probably won't contribute any longer to the Epic/Legendary badge, so take heart.