public void Protect( 
   WorksheetProtection protection,
   string password
)

using( var workbook = Workbook.Create( "test.xlsx" ) )

 {
   // Gets the first worksheet. A workbook contains at least 1 worksheet.
   var worksheet = workbook.Worksheets[ 0 ];

   // Sets content.
   worksheet.Cells[ "C6" ].Value = "This worksheet is protected with a password. Only formatting cell and inserting rows/columns is allowed.";
   worksheet.Cells[ "C8" ].Value = "The worksheet can be unprotected through the 'Review-Unprotect sheet' option by typing 'xceed'.";

   // Sets the worksheet protection : only formatting cells and inserting rows/columns will be allowed.
   var protection = new WorksheetProtection() { AllowFormatCells = true, AllowInsertRows = true, AllowInsertColumns = true };
   // Protects the 1st worksheet with a password.
   worksheet.Protect( protection, "xceed" );

   // Adds a 2nd worksheet.
   var worksheet2 = workbook.Worksheets.Add();

   // Sets content in 2nd worksheet.
   worksheet2.Cells[ "B5" ].Value = "This worksheet is NOT protected.";

   workbook.Save();
 }