I know the process of leaving an organization as a guest by going through myapplications.microsoft.com then going to organizations->Manage My Organizations. This works as long as you still have guest sign-in capabilities to the guest organization. If the guest tenant admin has removed sign-in capabilities, there does not appear to be a way to remove the guest organization. This means you will always have a "dead" guest in the drop-down company list. Is there any way to forcibly remove this "dead" tenant?