Destroying a specific session in Code Igniter

Create a new column in the ci_session table.

ALTER TABLE `yourdatabase`.`ci_sessions` 
ADD COLUMN `userid` VARCHAR(45) NULL  AFTER `user_data` ;

Then in your login function get the id from your login process and before adding the userdata information to the session do:

/* Delete any existing sessions with the current userid session.
Note - a new session has already been generated but doesn't have a value
in the userid column, so won't get deleted. */
$this->db->delete('ci_sessions',array('userid' => $identity));    

// Get the current session and update it with the userid value.
$session_id = $this->session->userdata('session_id');
$this->db->where('session_id', $session_id);
$this->db->update('ci_sessions', array('userid' => $identity));

Where $identity is your userid. That clears any previous sessions and means only one exists at a time.