They do provide a "soft limit" (which you get a notification when you reach) and a "hard limit" (which the system won't let you exceed) as options in your billi

They do provide a "soft limit" (which you get a notification when you reach) and a "hard limit" (which the system won't let you exceed) as options in your billing settings. I think by default they set the hard limit to $5, precisely to stop people racking up large bills by accident.