I hope that no one on here ever has to go through this kind of ordeal but here is what I did:
1. Wrote the withdrawal letter as suggested by milimelo and got it notarized.
2. Scanned the letter into a PDF file.
3. Mailed the original notarized letter to US Embassy with tracking service for delivery.
4. Sent the scanned PDF document via email to the US Embassy with the shipment tracking info. I had also requested them to send me a reply when they receive my email with attachment and they were nice enough to confirm the receipt.
The US Embassy accepted the scanned version of the letter and said they've added it to the file.
Thanks everyone.
ONN got a reaction from Monty and Lei in How to Cancel or Withdraw K-1 Petition?