The solution was in the latest examples, i must have been using an older version.
((HSSFSheet)hssfworkbook.GetSheetAt(0)).AlternativeFormula = false;
((HSSFSheet)hssfworkbook.GetSheetAt(0)).AlternativeExpression = false;
((HSSFSheet)hssfworkbook.GetSheetAt(0)).AlternativeFormula = false;
((HSSFSheet)hssfworkbook.GetSheetAt(0)).AlternativeExpression = false;