Yes, that appears to be the case.  As Listserv documentation takes some pains to stress, the mail client software will handle this.  In OWA (as an example), this looks to be user-configurable (set the "reply" default) -- unless you know the other is available (the two up-arrows show the other options) -- you are almost always going to go with the default.  So this is a known issue, and absent configuring everyone's mail client (as if) this is documented.  And it appears there is no way around it.  Thanks to every one who responded.
--Joe 

________________________________________
From: LISTSERV List Owners' Forum <[log in to unmask]> on behalf of Ben Parker <[log in to unmask]>
Sent: Saturday, July 2, 2016 3:48 PM
To: [log in to unmask]
Subject: Re: Force "Reply-to= Sender,Ignore"??

On Tue, 28 Jun 2016 15:28:44 +0000, F J Kelley <[log in to unmask]> wrote:

>UserA made a post to her list, it was distributed.
>
>UserB responded; it went to the list as well, in spite of the
>  Reply-To= Sender,Ignore

Simple.  As several people have noted UserB hit the Reply-All button (or
their email client defaults to this behavior).  This would cause a message
to go to every address in the list message.  The addresses available will
be:
 -the From: address (sender/UserA)
 -the Reply-To: address (also sender/UserA)
 -the List Address

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