You can set billing limits in the billing section of your dashboard.

You can access your billing settings at

Setting a Monthly Billing Limit

In your billing settings you can set a monthly billing limit:

Exceeding Billing Limits

Once you have used all of your starter credits, or exceeded your set monthly usage limit, you will start seeing errors in your dashboard & via the API mentioning Billing Limits Exceeded.

Once you have gone over on your monthly billing limits, the API & dashboard will throw billing-related errors.