Message box VBA - trigger to remain constant, need VBA to stop thereafter

Something like this:

Private Sub Worksheet_Change(ByVal Target As Range)
dim ans as String
If [A1].Value < 0 Then
      ans = MsgBox("Message here" & vbNewLine & vbNewLine & "Additional Message here", vbExclamation+vbOkCancel)
      If ans = vbOk Then
            ....
            ....
            ....
     End If
End If
End Sub

Sorry, there was one mistake, because I rather use Yes and No