Primefaces button ajax false

I have a button on primefaces:

<p:commandButton value="Exportar fichero" 
action="#{exportarView.exporta}"
ajax="false">

The method builds an excel and returns it, all this works well, the problem I have when doing a validation and not return the excel, that I have left the blank screen.

My Bean:

@ManagedBean(name="exportar")
@ViewScoped
public class Exportar implements Serializable {

    public void execute() throws IOException{

        if(validacion) {

        XSSFWorkbook workbook = new XSSFWorkbook(); 

        //Relleno excel ...
        FacesContext facesContext = FacesContext.getCurrentInstance();
        ExternalContext externalContext = facesContext.getExternalContext();
        externalContext.setResponseContentType("application/vnd.ms-excel");
        externalContext.setResponseHeader("Content-Disposition", "attachment; filename=\"Mh.xlsx\"");

        workbook.write(externalContext.getResponseOutputStream());

        facesContext.responseComplete();
        } else {
            //¿Que poner?
        }
    }
}

What should I put within else to return normally?