I was requested to do this the other day.. so I use the amazing MFCMAPI tool which opens wide mailboxes and thier raw content..
Be aware that i am really deleting an appointment ! the reminder will be gone with it. (Thanks for Steve for correcting me here.)
to do so we have a few steps:
1. Launch MFCMAPI tool on the user’s client.
2. Go to Session -> Logon and Display Store Table
3. Select the outlook profile of the user and double-click “Mailbox – your user name”
4. Expand “Root Container”
5. Expand “Top of Information Store”
6. Now its the tricky part , we need to find the specific reminder that does the troubles, there might be a few directory’s of reminders (maybe in more then one language…).
7. Double click the folder & start looking for the specific reminder (really the appointment)..
8. When you find the appointment or a few together even , right click and choose “Delete Message” , in the options select “permanent deletion (deletes to deleted item retention if supported)”.
9. close MFCMAPI & re-check in outlook.
you can use this method to delete problematic appointments, messages & all other MAPI data inside your exchange mailbox. BUT ! be careful , you can mess your mailbox up if you do something wrong …
Reference for steps and example …
Download MFCMAPI Latest version: