Eliminate a specific session variable

Hi All- I'm playing around with an authentication system and I can't figure out how to "Log" someone out of the system. Wha tI would like to do is be able to check if the session variable $_SESSION["logged_in"] exists. But if I unset() that session variable is still exists but it seems like the variable is just set to blank so an isset() check of that variable returns true, so they are still "logged_in". Is there any way to kill a single session variable using destroy() perhaps or will that nuke the entire session? Thx- Matt