Hey nate,
The nominations reset each year, so you will have to reapply again for congressional nominations. As far as denying the nomination, I don't know if someone else will be able to accept it at this point. For what its worth, USAFA and other military academies really like reapplicants.
Does it specifically say your application is closed? My application is listed as incomplete because I have not sent in college transcripts. Others have also yet to submit CFA scores and teacher recs. All the same, we are still in the running. I would double check with admissions just to see if you can finish whatever was left. Couldn't hurt to ask.