How to modify cell values from CellValueHandler


I have following requirement.
In spreadsheet if a user enters a negativa value inside a cell I need to show a message and reset cell value to 0.
To achieve this I have immplemented CellValueHandler and registered it with my spreadsheet instance. But inside cellValueUpdated() method if I change the cell value to zero it is not reflecting in UI. Is this a bug? Here is the code snippet of what I am trying to do

    public boolean cellValueUpdated(Cell cell, Sheet sheet, int colIndex, int rowIndex, String newValue,
            FormulaEvaluator formulaEvaluator, DataFormatter formatter) {