If you do not want to let others edit or modify the worksheets, you can lock them by password as below code:
Workbook workbook = new Workbook();
workbook.LoadFromFile(@"D:\facebook.xls");
Worksheet sheet = workbook.Worksheets[0];
Worksheet sheet1 = workbook.Worksheets[1];
sheet.Protect("123");
sheet1.Protect("456");
workbook.SaveToFile("Protected.xls");
System.Diagnostics.Process.Start("Protected.xls");
but if you do not want to let others see the worksheet, while not want to hide it, I think the best way is to copy these sheets to another new excel sheets and encrypt the whole excel file,and delete the sheets in the original excel. you can see the detail here in source code page: http://spreadsheet.codeplex.com/