How do I only show the job titles of unfilled jobs

We want to use the form generated in the Candidates Section. As this will add unassociated candidates, we want to add a 'Job applied for' dropdown. In the Customising Candidate Form, I created a lookup field with Job Opening and Posting Title fields.

This shows all jobs whether they are filled or unfilled. Obviously I don't want people applying for filled jobs!

Is there a way to show only unfilled jobs or can we actually use this form to automatically associate candidates to a particular job? We want to use the Candidates form because we prefer the emailing options.

Thanks

Graham