Spoke with Dish this morning.
You are correct, they will not allow you to transfer leased boxes between accounts.However
, you can easily purchase the box from them, and then transfer it to another account. I just confirmed this with Dish.
The purchase price depends on how long you've had it, but since it's a 211 and not a 211k I'm sure that's several years at least.
If you find out the purchase price, I'd be willing to pay you that in advance before you incur the cost, to make sure that you don't get stuck paying for something you don't want.
As for your original question, I don't have direct experience returning a modded box.
If you decide to return it, I would simply remove the circuit board (it's just held on with screws), and cut the ribbon cables close to the main board.
I don't think anyone would ever notice the hole in the back, as long as there is no mini-USB plug in it