I would discourage removing this header. This header can be exteremely useful when trying to debug mail list problems or when determining what list someone might want to be removed from.
It will aslo possible cause some delays in troubleshooting problems as you will not have the name of the mailling list them message came from available to you in the message.
However, if you still want to do this, look into using the headertrim keyword on the destination channel. Most likely the ims-ms channel.
You can find documentation on this keyword and how to set it up here:
http://docs.iplanet.com/docs/manuals/messaging/ims51/rm/mta_conf.htm#10492