Change your list header keyword Reply-to=    to:
* Reply-to= List

This won't gurantee that all replies go only to the list, but it will make
it more likely that those who blithely hit their "reply" buttons without
paying any attention to where the reply actually is sent will send their
replies only to the list. Their is no accounting for what some mail
systems, or their users, will do.

From what you say, it sounds like maybe you have either
* Reply-to= Both
or
* Reply-to= Both,Ignore

Douglas [log in to unmask]