As mentioned you need to contact Optus directly and follow up (unfortunately). Presumably you got no response to your email? I would suggest you write. Although old fashioned its handled more seriously than other methods. Just formally request the account be closed (take a photo).
I would also include a copy of the email and the date and request any payment since then be re credited. Optus can confirm you haven't been using any data in that time.
PO Box 306 Salisbury South 5106 SA
