No, this message is not customizable.

On Wed, 17 Mar 1999 13:01:13 -0600 Tim Ramsey said:
>Hiho,
>
>Is it possible to customize this message:
>
>  You are not authorized to send mail to the FOO list from your [ ... ]
>
>This is the message returned when someone tries to send mail to a list
>with restricted Send= access.
>
>Tim Ramsey / Sr. UNIX Admin         [log in to unmask]    (work)
>Enterprise Server Technologies      [log in to unmask]  (personal)
>Computing and Network Services      (785) 532-3742 (office)
>Kansas State University