Make Outlook AppointmentItem Read-Only - vba

Question

I am creating outlook appointment items programmatically using VBA in MS Access and the Outlook Object Model (though the language shouldn't matter).
Items are added to multiple calendars belonging to a single user that other users are given read/write permissions to. The users have no reason to create or edit appointments on the calendar using Outlook. Appointment data is then stored in backend tables. Essentially, Outlook is being used as my "calendar view."
I am having major issues, however, with users changing appointment items directly in Outlook, which in turn do not update in my backend.
I would love an updateable "ReadOnly" property that can be set per appointment item and that disallows changes unless set back to False...but don't think one exists. Any suggestions?
Things I've tried or dismissed as solutions:
Reminding users of the rules.
Script that finds all mismatched items - this works but is not practical.
Custom Outlook form that doesn't allow edits - doesn't prevent users from dragging appointment around.
UPDATE:
Using the suggestion by nemmy below, I have manged to get this far. This only works if the user selects the appointment before changing anything. It does not work if the appointment is selected and dragged in the same click.
Private WithEvents objExplorer As Outlook.Explorer
Private WithEvents appt As Outlook.AppointmentItem
Public Sub Application_Startup()
Set objExplorer = Application.ActiveExplorer
End Sub
Private Sub objExplorer_SelectionChange()
If objExplorer.CurrentFolder.DefaultItemType = olAppointmentItem Then
If objExplorer.Selection.Count > 0 Then
Set appt = objExplorer.Selection(1)
End If
End If
End Sub
Private Sub appt_Write(Cancel As Boolean)
If Not appt.Mileage = "" Then 'This appointment was added by my program
MsgBox ("Do not change appointments directly in Outlook!")
Cancel = True
appt.Close (olDiscard)
End If
End Sub

Answer 1

Can you hook into the Write Event of appointment items? You could prevent changes being made that way. Something like below might work (Disclaimer not tested):
Public WithEvents myItem As Outlook.AppointmentItem
Sub Initialize_handler()
Set myItem = Application.GetNamespace("MAPI").GetDefaultFolder(olFolderCalendar).Items("Your Appointment")
End Sub
Private Sub myItem_Write(Cancel as boolean)
Cancel=true
End Sub

Answer 2

The problem you have is that the people have write access, can you or is it possible to only give them read access? If that is not acceptable, then my answer is you cannot or should not stop them from changing the items. You need to deal with it. This is what I do.
So when you create the calendar item give it a unique ID for example the ID from your backend table row. Add this to a property of the calendar item like the mileage property.
Now just create an update method which loops through all current calendar items in your table and get them from outlook with the ID, check it has not changed and if it has update your table.
Alternatively and given your comment below;
In my opinion you must control outlook. As such nemmy is on the right track you need to hook into the outlook object probably with the use of an outlook addin. Then you need to get each appointment item that the user opens and check if it has a mileage ID. If it does you either need to tell them to change this in your data base and not outlook, or you need to get the events relevant indicating changes to the appointment item and wait for it to be changed. Then send these changes from outlook to your database.