There's 2 ways to do this:
1) Use a rule to run a script (How to create a script for the Rules Wizard
in Outlook:
http://support.microsoft.com/default.aspx?scid=KB;en-us;q306108). When your
function fires, you'll have to call
NameSpace.GetDefaultFolder(olFolderCalendar) to get a MAPIFolder object.
Then use Find or Restrict with MAPIFolder.Items to look for the Appointment
Item that was created in the Calendar as soon as the message was delivered to
your Inbox. You'll probably have to compare not only the Subject properties,
but also Start and End to guarantee you get the correct event. Once you have
the AppointmentItem object, set ReminderSet to False and Save.
2) Instead of a rule, monitor the ItemAdd event for your Calendar. You can
do this by grapping the Items collection object for the Calendar during
Application_Startup, and use a module level var for Items that uses the
WithEvents keyword so you can trap the event. When the event fires, you can
clear ReminderSet for all new appointments, or just for ones that have not
been accepted or rejected.
Let me know if you have any questions.