function pageLoad() {
    Sys.WebForms.PageRequestManager.getInstance().add_initializeRequest(cancelPostBack);
}

function cancelPostBack(sender, args)
{
    if (Sys.WebForms.PageRequestManager.getInstance().get_isInAsyncPostBack())        
    {
        //alert('One postback at a time please');
        args.set_cancel(true);
    }
}