How to ignore .qmail-* files for given users?

[John Chronakis]

Hello,

I would like to prevent users with ftp access to change the
delivery instructions for qmail.

To be more specific, I want some users not to be able to receive mail at all
(they all bellong to the same group) and others not be able to change
delivery instruction by creating their own dot-qmail files via ftp
(they all belong to another group).

I suppose that this is controled by QMAILHOME/users/assign,
but the man page is not very helpful.

Can anyone help?

John