7.4 Alter User xxx VALID UNTIL '2000-01-01' does not work

I'm trying to keep a user around but make it invalid.  E.g., by setting the
valid until to be a time in the past.  I've tried this:

    alter user pw_config valid until 'Jan 31 2001';

and this:

   alter user pw_config valid until '2000-01-01';

and even this:

    alter user pw_config valid until '-infinity';  -- for good measure

and none of them seems to have an affect.  The field is changed (pg_admin
says so) but psql still lets the user connect and execute statements.

Help!

  == Ezra Epstein