There's a stackoverflow post on this subject from a year ago: http://webapps.stackexchange.com/questions/12937/how-to-dele...

From the comments there, it sounds like the real API might limit you to deleting messages created by your own app. Somebody also posted a Javascript solution where you double-click the 'delete' button to delete all older posts, but I'm not sure if it will work since the version information says "obsolete" in it.