Postgres 7.4: how to disconnect users without restarting postmaster

Hi,
I have the following requirements, and I want to do them without shutting down postgres. Any help appreciated.

1. Get a list of connections to the postgres server
2. Disconnect some/all connections from postgres

3. I modified the pg_hba.conf to block a particular user from connecting to the DB. Then I issued the 'pg_ctl reload' command to send a SIGHUP to the server. After this I see the following:
a. Any new connection request from the user is blocked
b Any existing connection from that user (that was already opened, and is being pooled) doesn't get rejected.
Is (b) the expected behavior? How do I force existing connections from the user to be dropped without restarting db?

thanks
mlal

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