Answered question
Can I block data usage once I have no data left (to prevent fees)
When you have no data left in your plan, you will not be able to access internet until the beginning of the next cycle.
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Exception : if you put some funds in Wallet, Fizz will allow you to continue to use internet and will charge you 0.10 / mb. If you don't want surprises then don't put money in "Wallet"