Just to explain your situation:
You did NOT buy a FritzBox from Vodafone but you rented one -- this is a crucial difference.
Your monthly payment for this box is NOT a rate payment for the box but the rental fee - and as such you do NOT own the box but the box is still owned by Vodafone.
Thus you have to return the box at the end of your contract - if you don't return the box, Vodafone will force you to pay a non-return fee, but this does not constitute that you legally bought the Box then (Compare it to a rental car - if you don't return it on the end of the rental period and just pay the non-return fee, you also haven't bought the car).
And due to this fact: If you plan to not return the box at the end of the rental period / end of contract, you will not be able to re-use the box as Vodafone will internally mark this box as "lost/stolen" and thus will block the activation process for the box for the future. The box thus will become worthless for you - similar to a rented car not returned at the end of the rental period.
And this is also the reason why there is no menu to de-brand the box - as the box will become unusable if you don't return it.
