sessions managment

session.gc_probability = 0
1) You have garbage collector turned off; based on your settings.
You got an Ubuntu server? Look and see that you got a cron job running.
/etc/cron.d/php5

2) You need to set the following to limit the login to just one domain (or subdomain).
/application/config/config.php
$config['cookie_domain'] = 'subdomain.domain.com';
$config['cookie_secure'] = TRUE; // You are using https right?

a)
1. Lowercase this
$config['sess_cookie_name'] = 'myappname';

b) One user can only be logged in from one computer? Then you need to use database storage instead of files. And query the database and see if there are any open sessions for that user and destroy it.
(If that's the case, don't do that... I'm logged in on some websites from 3 devices)