Can't get rid of obsidian grief on xbox 360 minecraft?

I had a huge castle and someone built a obsidian wall around it that each side is 500 blocks long and 100 blocks tall what's the fastest way to get rid of it?

If he has been on your world long enough to build that and get away with it, then there's nothing you can do

Get a lot of people to help tear it down in creative mode, other than that there's nothing you can do if you saved the world