Password protect the generated Excel sheets

Question

My code generates an OpenXML spreadsheet, I want to protect against modifying and deleting operations. My solution seems working, but I found that opening it in the Excel, in the "Review" panel I can push the "Unprotect" button, and Excel unprotects the selected sheet without asking any password. How can I avoid this?

String password = Guid.NewGuid().ToString().Replace("-", string.Empty).Substring(0, 16);

using (SpreadsheetDocument excelDoc = SpreadsheetDocument.Open(input, true))
{
    foreach (WorksheetPart worksheetPart in excelDoc.WorkbookPart.WorksheetParts)
    {
        SheetProtection sp = new SheetProtection()
        {
            Sheet = true,
            SelectLockedCells = false,
            SelectUnlockedCells = false,
            Password = password,
            Objects = true,
            Scenarios = true,
            // AlgorithmName = "SHA-256"
        };
        foreach (OpenXmlElement currentChildElement in worksheetPart.Worksheet.ChildElements)
        {
            if (currentChildElement is SheetData)
            {
                worksheetPart.Worksheet.InsertAfter(sp, currentChildElement);
                worksheetPart.Worksheet.Save();
            }
        }
    }