Is there a cloud provider that does have hard unbreakable billing caps? Everything I've seen has always been notifications or soft caps.

Not talking about fixed-access things like a Hetzer box.

Bunny.net purports to have a pay-as-you-go prepaid credit system that sounds like it works the way people want, and with their description of the way it works probably being sufficient to be legally enforceable if it turns out that it actually works differently and you were to end up with a surprise bill from them. And evidently it really does work that way; see this post from a couple weeks ago: <https://news.ycombinator.com/item?id=47676416>

The only other provider known to work that way is NearlyFreeSpeech.NET, which serves a completely different market segment (so much so that it might as well not even be considered the same kind of product/service).