May I ask, How you are generating the Spreadsheet,What I mean by that is, are you writing each cell in a loop? If so, then you can set the DataType of each cell as string.
Cell cell = new Cell();
cell.DataType = CellValues.String;
cell.CellValue = new CellValue(table.Rows[rowIndex][colIndex].ToString());
than you can append the cell to other row cells.
Hope that helps !