The only way (other than not using all your data) is to change to one of the new plans that has "Peace of Mind" data (ie unlimited amount, but only at 1.5Mbps once you hit the monthly allowance) instead of automatic top up.
Depending on your device, you can go in to the mobile data settings and set a warning and/or limit on mobile data usage that can help stop you from going over if changing plans is not an option at this time.
Never be afraid to back yourself when trying new things, just always make sure you have 3 escape routes if things go wrong.
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