It's not quite that easy. It works for stateless services like transfers that can be just stop doing whatever they're doing. But presumably you don't want AWS to start deleting S3 buckets if a threshold is reached.

I actually tend to agree that, especially for hobbyist use, an automated hard cut off that cut out most further AWS service use would probably be desirable--even if some would (mis)use it in production environments and end up blowing up their site and complaining about it. I'm sympathetic to those who find the potentially open-ended nature of AWS billing to be bothersome. An alert is just an alert. There's no guarantee you'll be in a position to receive it and act on it in a timely manner.

Set up a lambda function that looks up billing. If billing reaches certain threshold, start shutting down resources. We do that and it works just fine.