I didn't see any way to tell Emacs to stop notifying me about an upcoming appointment after I've seen the notification, without turning off notifications for all appointments. So, I wrote this, which just removes the appointment from the appointment list, turning the alarm off.
(defun john/appt-notified () "Don't display any more notifications for the current appt." (interactive) (if (null appt-now-displayed) (message "No pending appt.") (pop appt-time-msg-list)))