[eluser]CroNiX[/eluser]
Your sessions are set to expire after 2 hours of inactivity. That means they go into a garbage collection system which may or may not immediately remove them.
You can always manually delete them from your session table if last_activity > 24 hours from current time and put that in a cron job that will run once a day or something. Is there a reason why you want to remove them? They will eventually be removed by the garbage collection.