What a pain! I've escalated your thread to grab the attention of a Optus rep to help out. In the mean time you could try web chat via the following link.
Call Optus on +61 2 8082 5678 (or livechat as provided)
Bar the Service
Block the IMEI number
Cancel any auto recharge.
You should be able to source a cheap local sim with some data (but only a local number).
Hey Matteo, sorry to hear that and apologies for the delayed reply.
Have you since been able to report this to our Customer Care team to have your SIM card blocked as a result of this?
You're welcome to PM me if you're still needing a hand with this.