To disable all the 'spellcheck', just override the 'initValue' method.
<script type="text/javascript">
(function($){
var initValue = $.fn.textbox.methods.initValue;
$.extend($.fn.textbox.methods, {
initValue: function(jq, value){
return jq.each(function(){
initValue($(this), value);
$(this).textbox('textbox').attr('spellcheck', 'false');
});
}
})
})(jQuery);
</script>
You also can disable the 'spellcheck' on the <form>.
<form spellcheck="false">
...
</form>