This method will guide you to add Disable Forwarding and Enable Forwarding buttons on the Ribbon in the Meeting window by VBA, and then you can prevent attendees from forwarding your meeting by the Disable Forwarding button. Please do as follows:

1. Press Alt + F11 keys to open the Microsoft Visual Basic for Applications window.

2. Click Insert > Module, and paste below VBA code into the new Module window.

VBA: Prevent attendees from forwarding meeting

Sub  ItemDisableForwarding()

  Dim xCurrentItem As Object

  Set xCurrentItem =  Outlook.ActiveInspector.CurrentItem

  xCurrentItem.Actions("Forward").Enabled = False

  MsgBox "Forwarding  current meeting has been disabled. Any meeting attendee is prevented from  forwarding this meeting."

  End Sub
Sub ItemEnableForwarding()

  Dim xCurrentItem As Object

  Set xCurrentItem =  ActiveInspector.CurrentItem

  xCurrentItem.Actions("Forward").Enabled = True

  MsgBox "Forwarding  current meeting has been enabled."

  End Sub 

3. Save the VBA code, and close the Microsoft Visual Basic for Applications window.

4. Open a calendar folder, and click Home > New Meeting to create a new meeting.

5. Click File > Options to open the Outlook Options dialog box.

6. In the Outlook Options dialog box, click Customize Ribbon in the left bar, click to highlight Appointment in the right box, and click the New Group button. See screenshot:

7. Now a new group is added under Appointment tab. Please click the Rename button, type a new name for it and click the OK button in the Rename dialog box. See screenshot:

8. Go ahead to (1) select Macros from the Choose commands from drop down list, (2) select Project1.ItemDisableForwarding in the left box, and (3) click the Add button.

9. Now the selected command is added to the right box. Keep it selected, and: (1) click the Rename button, (2) in the Rename dialog box, specify an icon for the command, (3) type a new name for it, and finally (4) click the OK button. See screenshot:

10. Please repeat above Step 8-9 to add the other Project1.ItemEnableForwarding to the right box, and click the OK button in the Outlook Options dialog box.

Now you return to the Meeting window, and you will see the two commands have been added under the Meeting tab. See screenshot:

11. Click Meeting > Disable Forwarding, and then click the OK button in the popping out dialog box. See screenshot:

12. Add meeting elements as you need, and click the Send button to send it.

If attendees want to forward this meeting by Meeting > Forward in their Outlook, a dialog box will come out to prevent the forwarding action.

Notes:
(1). This method won’t prevent attendees from forwarding the meeting as iCalendar attachment.
(2). This method  works well for Exchange account.

Actually, you can modify the meeting form and disable the Forward feature in it. All meetings created based on this form will not be allowed to forward in Outlook.

1. Open a calendar folder, and click Home > New Meeting to create a new meeting.

2. In the Meeting window, please click File > Options.

3. In the Outlook Options dialog box, please click Customize Ribbon on the left bar, check Developer in the right box, and click the OK button. See screenshot:

4. Now return to the Meeting window, please click Developer > Design This Form. See screenshot:

5. Enable the (Actions) tab, and double click the Forward row to open its Form Action Properties dialog box.

6. In the Form Action Properties dialog box, please uncheck the Enable option, and click the OK button. See screenshot:

7. Click Developer > Run This Form. See screenshot:

8. Now a new meeting is created based on the modified meeting form. Please add meeting elements as you need, and click the Send button.

Notes:
(1) If you need to apply this modified meeting form frequently, it’s recommended to save it by clicking Developer > Publish > Publish Form.
(2) All meetings created based on the modified meeting form are not allowed attendees to forward. However, attendees are not prevented from forwarding these meetings as iCalendar attachments.
(3) This method works well for Exchange account.

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