Unfortunately since March 25th Bungie completely shut down unlink system, they're not even able to unlink accounts throught support making no exceptions. This case of yours is a perfect case about how the actual policy do prevent fraud and illicit activities but don't protect users from mistakes and reverse decision. I have too a Steam account that I don't use anymore and I've decided to delete. Before it, I took a look to every services I have connected it, and Bungie.net is the only one who don't let in any way an unlink option. They should somehow review and reconsider actual policy and implement a better system like cooldown (like 12 months) between a linking or unlinking, like most other services already do. This would keep trust from their users.