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.
You could do it with the real API, but it could also probably be done with a few lines of Javascript pasted into the URL bar on your profile.
Any takers?