How to implement: Some items of dropdown are not valid if selected (Using jquery validation)

I have a drop down in my form.

I don't want my form to be submitted when a particular item in my dropdown is selected (based on some remote logic that I'm handling manually using some Ajax calls).

I prefer not to use jQuery validation remote rule, because of it's async nature (some times form is submitted when the remote rule is still working: Using jQuery validator's valid() or element() method with remote methods).

Where should I start? Do I need to add a custom rule?

jQuery Validate Plugin - How to create a simple custom rule?

or just adding my errors is sufficient?

$('form').validate().showErrors({
  "firstname": "I know that your firstname is Pete, Pete!"
});

or something else?