List to list to list forwarding

I inherited a list sometime ago and the previous admin forwards (automatically) the email from the old list address to the new, just like the previous list admin for him. I would really like to get the forwarding to stop as some of the is handled over 3 hops and I have not been able to reach any of the previous admins. Is there an easy way to reject the message and send a reply to the sender to use the new address. I suppose something in the rc.local.s00 to check the header and the use the commonly found rejectmsg script?