overwrite permissions on default tablespace

Hi PGadmin list,
I would like to know how can I overwrite the existing permission( on a
default table space) defined in postgresql. cfg with :"set
default_tablespace =x" for a user, trying : "revoke all on tablespace
x from user" doesn't seems to help , the session level "set default ..."
is not to 'satisfactory' approach for us.
Thank you
Isabella

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Isabella A. Ghiurea

Isabella(dot)Ghiurea(at)nrc-cnrc(dot)gc(dot)ca
Canadian Astronomy Data Centre |http://www.nrc-cnrc.gc.ca/eng/services/hia/data-centre.html
National Research Council of Canada, Herzberg Institute of Astrophysics
5071 West Saanich Road, Victoria BC V9E 2E7, Canada
Phone: 250 363-3446 fax: 250 363-0045