It is finally possible to delete all the files in one go using the new Lifecycle (expiration) rules feature. You can even do it from the AWS console.
Simply right click on the bucket name in AWS console, select "Properties" and then in the row of tabs at the bottom of the page select "lifecycle" and "add rule". Create a lifecycle rule with the "Prefix" field set blank (blank means all files in the bucket, or you could set it to "a" to delete all files whose names begin with "a"). Set the "Days" field to "1". That's it. Done. Assuming the files are more than one day old they should all get deleted, then you can delete the bucket.
I only just tried this for the first time so I'm still waiting to see how quickly the files get deleted (it wasn't instant but presumably should happen within 24 hours) and whether I get billed for one delete command or 50 million delete commands... fingers crossed!
