Set problem child to REVIEW e.g.,
 
QUIET SET listname REVIEW FOR [log in to unmask]
 
This allows the list-owner to "moderate" just that single
subscriber's e-mail.
 
Furthermore, depending upon the frequency of new e-mail
subscriptions, you might consider * SUBSCRIPTION= BY_OWNER
 
If your dialog with problem child continues to be fruitless,
you might consider escalating it with his/her management
or ISP.
 
As always, please make sure your Welcome file is explicit
on such matters.
 
/Pete Weiss -- Penn State