Well, this is dangerous because there will be no opportunity to save the
file under another name if it's read-only. But, assuming that's what you
want to do, remove the read-only property, open the file, right-click on the
Excel icon at the left of the toolbar and select View Code. Copy this code
and paste it in there. Save the file, close it and set it to read-only.
HTH, James
Private Sub Workbook_BeforeClose(Cancel As Boolean)
If ThisWorkbook.ReadOnly Then ThisWorkbook.Saved = True
End Sub