That is kind of what the throttle feature does. You could have the ‘Bandwidth Exceeded’ page setup in the throttle.yourdomain.com directory. Obviously, the method is not perfect, for one thing, the bandwidth usage is only checked every 24 hours and the throttle.yourdomain.com directory will be accessible by whichever user ‘owns’ the domain.
Unfortunately I know of no other way to implement a bandwidth limit for a domain, sorry.
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