Hello! We don’t have any way to limit billing to $0. With the exception of bandwidth, you only pay for what you manually provision. So anything paid is intentional. This isn’t a pay-per-request product where 10 million people show up and you get a big bill at the end.
We do have prepaid support. You can buy $25 or more of credits and we suspend the account when credits are exhausted.
For what it’s worth, we probably won’t build a way to prevent accounts from using paid services. We pay the cost to run free services with the hope that people will deploy projects that are worth paying for when we’ve proven that what we’re doing is valuable.