Locking cells in a JXL-generated Excel document

Question

Is there a way to lock certain cells from being edited by the user? I have tried the following:

WritableCellFormat cf = new WritableCellFormat();
cf.setLocked(true);
sheet.addCell(new Number(col, row, value, cf));

However this doesn't seem to have any effect on the resulting spreadsheet i.e. the cell contents can still be changed when the file is opened in Excel.