How do you make a workbook or worksheet read only using SpreadsheetML / Excel 2003 XML?

Question

I am using SpreadsheetML to generate an Excel spreadsheet with multiple worksheets.

I would like to make the document read only when opened with Excel. How can I do this?

(I appreciate that a user could open the XML file as text and do what they want with it so this is read-only in the loosest sense.)

Thanks

Spreadsheet ML references:

https://en.wikipedia.org/wiki/Microsoft_Office_XML_formats#Excel_XML_Spreadsheet_example

http://m8y.org/Microsoft_Office_2003_XML_Reference_Schemas/Help/html/spreadsheetml_HV01151864.htm

https://en.wikipedia.org/wiki/SpreadsheetML