How could I stop spam if my form is using ajax submit method (with no reload)?

I have a form which uses ajax for data submitting. Here is the html:

<form>
    <input type="text" id="content" name="content">
    <input type="submit" class="button" name="button">
</form>    

and the JavaScript (jQuery):

$(function() {
    //Update Message...
    $(".button").click(function() {
        var boxval = $("#content").val();
        var dataString = 'content='+ boxval;

        if(boxval=='')
        {
            alert("Please Enter Some Text");
        }
        else
        {
            $("#flash").show();
            $("#flash").fadeIn(400).html('<img src="ajax-loader.gif" align="absmiddle"> <span class="loading">Loading Comment...</span>');

            $.ajax({
                type: "POST",
                url: "update_data.php",
                data: dataString,
                cache: false,
                success: function(html){
                    $("ol#update").prepend(html);
                    $("ol#update li:first").slideDown("slow");
                    document.getElementById('content').value='';
                    document.getElementById('content').focus();
                    $("#flash").hide();
                }
            });
        } 
        return false;
    });
});

It is all working fine, but there is a big problem: I cannot stop spam. Last time I used spam filters, it was in PHP (I used a hidden input and then checked in the PHP file if it is filled out). But that was totally in PHP, no JS.

So here is 开发者_开发问答my question: How could I stop spam in this form submission? Also, can I create an if statement in the update_data.php file?

---

I you do not like to add Captcha here is a good solution:

First add input that is hidden with css in the form. If the bot submits the form the hidden field will have some value filled by the bot. In normal cases the human will not see it and will be empty. In the case of a bot you will check it and if it has some value you will discard. Also you need to add input (also hidden with css) with javascript on document ready. If it is browser it will execute the javascript and will insert the input in the form. If it is a bot then the bots do not execute js and there will be no input field generated by js and on the server side you will see that there is no such field and you will discard the request.

So on the server side you will check if the first hidden field has value or the second hidden field does not exist discard the request.

---

First fast answer is putting inside the form something that could help you to understand if the user submitting your form is a bot or not.

Captcha for sure, or just a question not easy to answer for a bot.