I believe the latest code can do traffic shaping which will probably be your best approach.
You can traffic shaping the ingress/egress ports to the specific customers and apply traffic shaping to those ports to keep the traffic within some range. I haven't set it up personally (I've done it on and ERS 8600 switch pretty easily) on an ERS 5500/5600 series switch.
There might be a technical configuration guide available.... did you give a look at Nortel's old support site?
Cheers!