Punishment for refusing NJP? It's your right (in most cases) to refuse NJP. Then it is your command's option to drop the whole business (unlikely) or to take the offense to a Summary Court-Martial, Special Court-Martial, or General Court-Martial. In general, as you move up to the various Courts-Martial the requirements for them to prove an offense become more strict and the maximum penalty that may be imposed also increases.
Before making this decision you should get legal advice from an attorney versed in military law.
As I recall, once you refuse NJP and the command says, "OK, we're going for a Court-Martial", you don't get a do-over. You don't get to say "Oh, sorry, I'll take that NJP now." Well, actually you could 'say it' but it won't change anything.