So should the OP start charging them storage fees? The statements "They don't want it back" and "You're not allowed to sell it or dispose of it because it isn't yours" seems to be at odds with one another. If you don't own it, they need to retrieve it.
Oh the OP can totally dispose of it or recycle it. D* will even tell him to dispose of it or recycle it. It just can't be activated on another account if it changes hands.