To kill only one connection use this:

SELECT pg_terminate_backend( process_id )

where “process_id” is taken FROM pg_stat_activity – procpid in PG 8.4 or pid in PG 9.3


If you need to terminate processes / connections / sessions on some specific database you can use this:

SELECT pg_terminate_backend(procpid) 
FROM  pg_stat_activity 
    -- don't kill my own connection!
    procpid <> pg_backend_pid()
    -- don't kill the connections to other databases
    AND datname = '...database_name...';


Of course if you don’t case about some specific connection you can always restart pg server. Which is quick but ends everything…