How to validate data using beforeSend with Ajax

Question

I’m developing a messaging system with initial and final date of each message, so before sending the data via Ajax, I want to validate some fields. I did a good part, the only problem is that when I finish the validation the page is reloaded, preventing the user to perform the correction. Help!

form.ajaxSubmit({
        url: 'themes/intranet_fametro/ajax/' + callback + '.ajax.php',
        data: {callback_action:callback_action},
        dataType: 'json',
            beforeSend: function(){
                if (callback_action == 'criar-mensagem') {
                    if(dataInicial < dataAtual){
                        $("#load").html("Data inicial errada, insira uma data posterior.");
                    }
                    else if(dataFinal < dataInicial){
                        $("#load").html("Data final incorreta, insira uma data posterior.");
                    }

                }
            },
            success: function(data){
                window.location.href = 'mensagem';
            }

        });