Disabling constraints

Dear friends,

I am working opn Postgres 7.3.4 on RH Linux 7.2.

I wanted to disable constraints.

Alter table 'table name' disable constraint 'constraint name'; doesn't work.

I got some information from google, which says about indirect way of disabling and enabling a constraint, as follows.
update pg_class set reltriggers=0 where relname = 'crm.activities';
update pg_class set reltriggers = count(*) from pg_trigger where pg_class.oid=tgrelid and relname='crm.activities';

Also doesnt work.

Is there a way to do it?

Thanks
Kumar