[rt-users] prevent email ticket creation by new users?

[Phil R Lawrence]

By default, a new user is auto-created if an unrecognized email address 
is used.  How can I disable this?  I only want users that I create to 
have the right to create Tickets.

Yet if I take away the Everyone groups right to create tickets, I cannot 
grant that right to my users, because they do are not priveleged (to 
force them to the self service page)

What should I do?

Phil