how to hide work sheet in epplus in C#?

Yes, you can using the Hidden property of the worksheet:

ExcelWorksheet ddworksheet = package.Workbook.Worksheets.Add("DropDownList");
ddworksheet.Hidden = OfficeOpenXml.eWorkSheetHidden.Hidden;

I know this thread is old but for everyones help Hidden will not work if the sheet that you want to be hidden is active, and so use ws.select to select some other sheet before you save. Otherwise hidden will have no impact and only VeryHidden will work.