If you are sure that the value does not exist in the data list and you wish to clear the text box, please call 'setText' method with an empty string after calling 'setValue' method.
$('#Region').combobox('setValue', '5').combobox('setText','');
Another alternative way is to extend a new validation type to check if the value exists in the data list.
$.extend($.fn.validatebox.defaults.rules,{
inList:{
validator:function(value,param){
var c = $(param[0]);
var opts = c.combobox('options');
var data = c.combobox('getData');
var exists = false;
for(var i=0; i<data.length; i++){
if (value == data[i][opts.textField]){
exists = true;
break;
}
}
return exists;
},
message:'invalid value.'
}
});
$(function(){
$('#cc').combobox({
required:true,
validType:'inList["#cc"]'
})
});