[fallingsquirrel]

Of course there is. If someone hits their spending limit, asynchronously shut off the services (using the same API call that your customers can use, so no need to alter the system).

Then apply the hard limit in the billing code. If it took a minute or two to shut off all the instances, maybe the customer's bill should have been $1.001M instead of $1M, but cap the bill to $1M anyway. Given their profit margins of x,000% I think they can afford the lost pennies.