ASP.Net MVC Ajax form with jQuery validation

I have an MVC view with a form built with the Ajax.BeginForm() helper method, and I'm trying to validate user input with the jQuery Validation plugin. I get the plugin to highlight the inputs with invalid input data, but despite the invalid input the form is posted to the server.

How do I stop this, and make sure that the data is only posted when the form validates?

My code

The form:

<fieldset>
    <legend>leave a message</legend>
        <% using (Ajax.BeginForm("Post", new AjaxOptions
           {
               UpdateTargetId = "GBPostList",
               InsertionMode = InsertionMode.InsertBefore,
               OnSuccess = "getGbPostSuccess",
               OnFailure = "showFaliure"
           }))
           { %>
        <div class="column" style="width: 230px;">
            <p>
                <label for="Post.Header">
                    Rubrik</label>
                <%= Html.TextBox("Post.Header", null, new { @style = "width: 200px;", @class="text required" }) %></p>
            <p>
                <label for="Post.Post">
                    Meddelande</label>
                <%= Html.TextArea("Post.Post", new { @style = "width: 230px; height: 120px;" }) %></p>
        </div>
        <p>
            <input type="submit" value="OK!" /></p>
    </fieldset>

The JavaScript validation:

$(document).ready(function() {
    // for highlight
    var elements = $("input[type!='submit'], textarea, select");
    elements.focus(function() {
        $(this).parents('p').addClass('highlight');
    });
    elements.blur(function() {
        $(this).parents('p').removeClass('highlight');
    });

    // for validation
    $("form").validate();   
});