New unified Windows Dev Center - how to unpublish/hide/delete app?
Select your app from the dashboard, go to submissions and click update to create a new submission (yes counter intuitive) and then go to pricing and availability and scroll down to Distribution and visibility and click on show options.Then select hide this app and stop selling, save it and submit it.
The new dev center is in preview mode(read only), so I think that you will have this feature when the account migrations are finished. Until then, you also can't publish new apps.You can find more info here: http://blogs.windows.com/buildingapps/2015/06/05/dev-center-account-migration-begins-next-week/