It actually does not look like it is possible at this point in time to disable the validator; however, you can override the select element to be able to remove the validator for this specific case:
use Zend\Form\Element\Select;
class MySelect extends Select {
public function getValidator() {
return $this->validator;
}
}
Basically the key issue with the current select element is that if the validator does not exist; it will create it. The other option you have here is to set a validator manually; which you should likely be doing is manually creating an InArrayValidator and populating it with the potential options that would be coming from your AJAX call. In which case you would need to add a setter above.