Visit: https://openai.com/api/pricing/
Expand: How can I manage my spending on the API platform?
Read:
You can set a monthly budget in your billing settings(opens in a new window), after which we’ll stop serving your requests.
The only way OpenAI “stops” is when they’ve emptied your entire credit balance (or with recharge, when they’ve maxed your card). The only facility offered now is an email alert, with a poor justification made for removing a hard limit developer setting that existed in the past.