well that got me stumped because that usually fixes a stuck notification. Try it once more and leave it out for about 30 seconds. If that still does not fix the issue, try to reload the os
problem i have with the Calander notifications, is that it tends to show multiple items of the same thing example it had a birthday listed in the notifcations area for a contact of mine 4 times..
"04/29 Chelsea Birthday"
"04/29 Chelsea Birthday"
"04/29 Chelsea Birthday"
"04/29 Chelsea Birthday"
so i went to the calander an opened the series and deleted it, since i don't need birthday reminders, and i did not set them up, i think it was from facebook when it imports the contact details, so after doing that,they no longer appeared on the calander however they still appeared in the notifications area, but when clicked on i get a message saying something like "appointement deleted externally viewer will now be closed", ok... so i wait, and wait.. and they don't disappear from the notification area unless i do a battery pull... kinda tired of doing battery pulls lol.