-- View
SELECT procpid, current_query FROM pg_stat_activity;

-- Cancel
SELECT pg_cancel_backend(enter procpid here);

OR

SELECT pg_cancel_backend(procpid)
FROM pg_stat_activity
WHERE usename = 'some_user'
AND current_query LIKE '<IDLE>%';

-- Cancel Multiple Sessions At Once (Pre-9.2 use "procpid" instead of "pid")
SELECT pg_terminate_backend( pid )
FROM pg_stat_activity
WHERE pid <> pg_backend_pid( )
    AND datname = current_database( );

-- Terminate
SELECT pg_terminate_backend(enter procpid here);

OR 

SELECT pg_terminate_backend(procpid)
FROM pg_stat_activity
WHERE usename = 'some_user'
AND current_query LIKE '<IDLE>%';