Outlook 2010 calandar error message "cannot turn off the reminder. you may be reminded again. the function cannot be performed because the message has been changed"

Begin by selecting the Calendar folder and then select View -> Current View -> All Appointments to see a list of all of your past, present and future appointments.
The trick now is to sort through the list until you find the damaged calendar entry. The corrupt appointments should be pretty easy to spot, because they will have the same names as the appointments that you see each time you open Microsoft Outlook.
Delete the corrupt Microsoft Outlook appointment.
You will see a dialog box filled with a series of error messages telling you that the appointment cannot be deleted, recurrence information cannot be located, and that the sender may not receive updates to the appointment. In spite of these errors, the Microsoft Outlook appointment will be deleted anyway.

In Outlook 2003, deleting a corrupt appointment doesn't always work. Sometimes the stubborn reminder would be displayed again upon restarting Microsoft Outlook. As I outlined though, there is a second workaround you can try.

This workaround does not seem to exist in Outlook 2007. At the same though, in spite of the error messages received, I have always been able to delete corrupt appointments on the first try in Outlook 2007.

http://www.slipstick.com/outlook/calendar/delete-reminders-that-keep-returning/