You can use pg_terminate_backend() to kill a connection. You have to be superuser to use this function. This works on all operating systems the same.

SELECT 
    pg_terminate_backend(pid) 
FROM 
    pg_stat_activity 
WHERE 
    — don’t kill my own connection!
    pid pg_backend_pid()
    — don’t kill the connections to other databases
    AND datname = ‘database_name’
    ;

Before executing this query, you have to REVOKE the CONNECT privileges to avoid new connections:
REVOKE CONNECT ON DATABASE dbname FROM PUBLIC, username;

If you’re using Postgres 8.4-9.1 use procpid instead of pid
SELECT 
    pg_terminate_backend(procpid) 
FROM 
    pg_stat_activity 
WHERE 
    — don’t kill my own connection!
    procpid pg_backend_pid()
    — don’t kill the connections to other databases
    AND datname = ‘database_name’
    ;