Icons remain in notification area if application gets killed

[Guido Günther <agx sigxcpu org>]

Hi,
I recently ported krb5-auth-dialog over to use persistent notifications.
I noticed that killing the app with CTRL-C leaves the icon and the
persistent notification in the notification area. The same is true for
rhythmbox. This is unfortunate since the buttons in the notification
obviously won't work when the app is gone which confuses users.
Is there a proper way to handle this?
Cheers,
 -- Guido