Eight, nine years ago it used to be a standard feature of "netiqutte"
that one does not talk religion, politics, sexual orientation,...
on a list or newsgroup not specifically dedicated to such topics.
These days I don't know how many people have ever heard of "netiquette."

Simply tell them that the purpose of the list is not to discuss religion
and it will not be tolerated as it leads to flame wars and you will not
tolerate flame wars (and that inculdes such things as "accept Jesus
and you will be cured.")

On one of my lists religion and philosphy must be discussed regularly
as it is for library cataloging.  Discussion of how to catalog the various
works is entirely appropriate.  However, said discussion must not digress
into promotion nor even mention, of one's personal religious beliefs as
they are totally irrelevant.  When a discussion shows definite signs of
skewing in that direction a firm reminder is injected that there
will be NO
discussion nor mention of religion as such
discussion nor mention of politics as such
discussion nor mention of sexual orientation as such
etc.

It usually works as they know we'll do something about it if the warning
is ignored.  usually takes only the one reminder.  The stuff is not
germane to the list and is just as much spam as a commercial spam.

Douglas