Anyway I can temporarily disable any further data from being used this month as during a recent blackout my cell data was mistakingly used and they keep automaticly topping me up for amounts of $10 which will take awhile to pay off.
The (unavoidable) auto top up system is one of the worst aspects of Optus IMO. You're exposed to up to $1500 a month.
Are you talking about a mobile phone or mobile broad band?
Legally Optus need to send you SMS when your data is 50, 80 and 100 and for every top up. Did they do this?
On your question you can ring Optus to have them turn your data on or off each time. But you can also turn off data on your device?
if it is a temporately thing for this month until your data is replenish, then you can turn off mobile data from your mobile. It is a setting on your phone, so when next month come, you will need to turn it back on.