VBA code for Office to monitor and automatically dismiss MessageBox windows that appear in Excel. Monitoring is performed from a separate Word document process.I did not write this code. The code was authored originally by A.S.H. and was copied from https://stackoverflow.com/a/41826510/22859713.

Excel_MessageBoxMonitor.bas

' Author: A.S.H; Source: https://stackoverflow.com/a/41826510/22859713
' Excel - Class Module VBA
Private killerDoc As Object

Private Sub Class_Initialize()
    On Error Resume Next
Set killerDoc = CreateObject("Word.Application").Documents.Open(Filename:="C:\BElston\Office\Custom\MessageBoxMonitor.docm", ReadOnly:=True)
    If Err.Number <> 0 Then
        If Not killerDoc Is Nothing Then killerDoc.Close False
        Set killerDoc = Nothing
        MsgBox "could not lauch The mboxKiller killer. The message-box shall be closed manuallt by the user."
    End If
End Sub

Private Sub Class_Terminate()
    On Error Resume Next
    If Not killerDoc Is Nothing Then killerDoc.Application.Quit False
End Sub

Excel_MessageBoxMonitorTest.bas

' Author: A.S.H; Source: https://stackoverflow.com/a/41826510/22859713
' Test MessageBoxMonitor
Sub Excel_MessageBoxMonitorTest()
    Dim killer: Set killer = New mboxKiller
    Excel_MessageBoxMonitorTest_Simulate
    Excel_MessageBoxMonitorTest_Simulate
    Excel_MessageBoxMonitorTest_Simulate
End Sub

Private Sub Excel_MessageBoxMonitorTest_Simulate()
    Dim i As Long
    For i = 0 To 1000: DoEvents: Next

    MsgBox "This is a message box to simulate the message box of the addin." & VbCrLf & _
           "It will be automatically closed by the Word app mboxKiller"
End Sub

Word_MessageBoxMonitor-docm.bas

' Author: A.S.H; Source: https://stackoverflow.com/a/41826510/22859713
' Word - ThisDocument VBA
Private Declare PtrSafe Function FindWindow Lib "user32" Alias "FindWindowA" (ByVal lpClassName As String, ByVal lpWindowName As String) As Long
Private Declare PtrSafe Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, ByVal lParam As Long) As Long

Public Sub WaitAndKillWindow()
    On Error Resume Next
    Dim h As Long: h = FindWindow(vbNullString, "Microsoft Excel")
    If h <> 0 Then SendMessage h, 16, 0, 0 ' <-- WM_Close
    Application.OnTime Now + TimeSerial(0, 0, 1), "WaitAndKillWindow"
End Sub

Private Sub Document_Open()
    WaitAndKillWindow
End Sub