The fact that AWS has no way to limit billing seems insane to me. Your only recourse for an accidental (or malicious) overcharge is beg customer support. It's an incredible liability.
Limiting billing is ambiguous. The computation spent is spent, unless AWS can predict how much you are going to cost by even initiating certain operation, which is not going to be straightforward for all the services they provided.
It's not hard to invert the transaction, ask the billing service if an action can be done instead of bill upon completion. Done. This isn't that hard. If your credits won't make it to the end of the month, send out the email warnings to top up - which should be predictable unless there's a large spike hitting the account. If you prefer invoice billing because you never want your services going down, it's trivial to accommodate with this system design.
Sure, big warnings about data loss/services down in the event of zeroing out your credits.
Having such a safety net would really help the thousands of people who sign up for transient reasons. I don't recommend a noob set up an AWS account. I also don't let my kids play on the freeway.