Hi,
You can try with the option provided by Lakshmipathi sir, however I have doubt that it will help, since you ahve already created the Invoice (only posting is blocked), so your Delivery status is already 'completed'.
Now we do not have any option to complete the Billing without posting the invoice. Since the invoice is blocked for posting so it will always appear in VFX3 also, and so your concern is that no one should release it. But since the invoice is of 2012, and as you said the FI Period is also closed, so I think that even if someone release it to accounting, system will give error since FI period is already closed. For extra pre-caution you may make the Customer Account Assignment group and Material Account Assignment Group 'blank' in your invoice by going to VF02 --> Header and VF02 --> Item. By doing this system will give error message of account assignment determination and not post these documents to accounting.
But these documents will always appear in VFX3.
Thanks and regards,
Amitesh Anand