Amazon sucks and they don't have any automatic way to shut down a service. I emailed them and they were very clear about that. The best you can do is set up an email alert at different cost threshold (which I have done). But that requires someone with credentials to be awake and online when it happens. This is the main reason I want to drop Amazon. (Apart from the billing concerns, Amazon's CDN, technically, is nice and fast and reliable.)
Would it make sense to add some code to your meek server to monitor bandwidth usage and automatically shut off if a limit is reached?