As with any password protected website, you should be careful about leaving There Central open when you are using a public computer, a friend's computer, or any computer that you share with someone else. If you are logged into There Central via There.com (i.e., you do not have the There client open), and you are finished with your session, simply close the browser window containing There Central, and you will be automatically logged out.
If you are logged into There Central via the client, you will be automatically logged out when you exit your session in There.