Yeah I also left my website hosted on Google Cloud because costs popped from everywhere, and there is basically no built-in functionality to limit them. So I didn't really slept relaxed (I actually slept great, but I hope you get the point) knowing that a bug could cost me... who knows how much. Actually, as the website of OP says, for spending control you have budget notifications and with that you can disable the billing for all the project altogether through some API call or something, I don't remember exactly, that is all there is. But still it looks like this functionality is just not there.
You can write Google cloud functions to disable your credit card when certain thresholds are met pretty easily, but it's unethical that this isn't just a toggle somewhere in settings.
Does that actually stop the spend immediately? If not, you're still on the hook for the bill. I suppose you can walk away and let them try to come after you, but that wouldn't work for a company.