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