deny table creation

Hello,

I am trying to deny the ability to create tables in a database to
all users except the database owner and the superuser.

As superuser, I have attempted to revoke insert/update/delete/rules
permissions to users on pg_class and pg_tables, but to no avail. It
would appear that the read/insert/update permissions on these two
relations are fixed. Users are always able to create tables.  Yet when
I try to display the permissions I get

 \z pg_class
Access privileges for database "cswdb"
 Table | Access privileges
-------+-------------------
(0 rows)

\z pg_tables
Access privileges for database "cswdb"
 Table | Access privileges
-------+-------------------
(0 rows)

Is it the superuser who is updating these tables via Rules rather
than the current user?

Can any-one suggest an alternative approach to preventing users
from creating tables?

Thanks
Maggie Bowman