Charlie,
Lets start and usethe tablet you are trying to remove...
In the iFlyGPS app go to 'Menu' ---> 'About' ---> 'My Account...' ---> 'Deregister a Device'
From here you need to select whichever has '(this device)' attached to the end of it. This will deregister the device you are currently on.
Hope this helps!
-JJ