What seems to have worked in the past is to sent a private message to one of the forum moderators (by clicking on their screen name. Moderators have a 'capital N' after their screen name where the rest of us have some goofy symbol.) Give enough details (that should never be posted in a public forum) for them to know exactly which item should be remove from which account.
I have often wondered about the reasoning behind not allowing users to 'deregister' devices. Maybe it's an anti-theft mechanism? Maybe so few people have a need to that they didn't find it worth doing? No idea.
