It's not really my area, I'm afraid, but I believe that the problem would be that they can't be deleted in the usual way that a reported thread would be.
If it was a normal thread, it'd just be a case of opening it up and pressing the delete button.
But since these threads can't be opened in order to do that, I imagine it would be a case of deleting directly from the database which will mean diverting development resources away from other important site maintenance.