I have been playing with the Mobile Device Management feature for Office 365 for some time already – since it has been in technical preview.
I registered many devices running on different OS (Windows, Windows Phone, Windows 10 Tech Preview, Android..) but recently I discovered a rogue/phantom device in the Office 365 Mobile Device Management section.
This device was unregistered some time ago already, has been restores and re installed to Windows 10 but was still present.
From the Azure Management Portal, I have not been able to delete (as this is usually the case) from the Azure Active Directory\User account\Device section. The delete button was greyed as well as all other buttons except the Block device.
After opening a service request, I have been provided a web site to get all the registered devices with an option to delete them and from this site I have been able to delete the phantom device
So, if you also have a similar issue with an “old” registered device but not able to delete it from the Azure Active Directory portal, go to this site https://byodtestservice.azurewebsites.net/, sign in with the credentials used to registered the device.
If the device is shown in the list, just hit the Delete action