I experienced a similar issue from Aug 12 to 24, where my Gemini API usage costs unexpectedly shot up to over $1,000. On Aug 20 alone, the charge was $1,700. I was completely shocked by this, as my project is just a personal one and the cost had always been $0 before.
To stop the charges, I deleted the entire project, changed my account password, and contacted support.
Support replied that they were aware of the issue and were working on it, and that it would take about 2-3 days. I really hope this gets sorted out soon. I had to lock my credit card because I thought my account had been hacked 
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