You can interrupt reply in a similar manner as described in this question
Insert a new module like this and then in the ThisOutlookSession module paste the below code
Dim WithEvents insp As Outlook.Inspectors Dim WithEvents mailItem As Outlook.MailItem ' This is called on Outlook startup Private Sub Application_Startup() Set insp = Application.Inspectors End Sub ' This is called when a new Inspector is created. Private Sub insp_NewInspector(ByVal Inspector As Inspector) If Inspector.CurrentItem.Class = olMail Then Set mailItem = Inspector.CurrentItem End If End Sub ' Called when you press Reply Private Sub mailItem_Reply(ByVal Response As Object, Cancel As Boolean) Dim msg As String Dim result As Integer If myMailItem.Recipients.count > 1 Then msg = "Do you really want to reply to one?" result = MsgBox(msg, vbYesNo, "Reply Check") If result = vbNo Then Cancel = True End If End If End Sub