@Anonymous User in theory it should be as simple as following the instructions in the green box in this link.
But, there was an issue with this system that got highlighted a few weeks back. I'm trying to find the thread that was started so I can link it.
In the mean time I know @RoyB was feeding the issue back to the powers that be so it can be resolved so if I can't find the thread he may be able to give an update.
@Anonymous User thread found.
My advice would be to try the procedure set out by Now in the original help article I linked.
If it doesn't work you'll need to do two things. Firstly jump on to live chat and see if they can permanently remove the device (link below). Secondly, let us know here so that a community manager can be notified that the problem persists.