why would you want to do that?? you've bought that bandwidth and you want to prevent it being used effectively? I think you quite possibly would be much better off looking at queueing methods to ensure priority to certain networks, and bursting features to allow better use of what you've paid for...
http://lartc.org/howto/lartc.qdisc.html Mind you, you can tie down the configuration of the queueing to do what you've asked, but I'd urge to to read about better ways to control the bandwidth.
