As far as I know, they implement billing alerts but, aside from some student and some other limited account types, they're alerts. You'll get an email that you've hit your limit but your bill will continue to go up until you shut things down.
And do note that these alerts are not instant. With Azure, if their backend reports and alerts are timed wrong, you're still on the hook for 2-3 days' worth of costs.
They have limits on things like how many EC2 instances you can have but not on things like bandwidth.
While you can raise those limits by request I'm also not sure whether you can actually reduce them again later.
Aws doesn't, there are only alerts. Don't know about others.