Do you have an HMO or a PPO? This matters since with a PPO they can try to deny the medication and tell you that you have to follow a specific path, but they can't go against your doctors wishes in the end. You might have to take remicade instead of humira since it has been around longer and is the standard of care in biologics for crohns right now, but that is a good choice as well and most people take that first anyway. If they do deny you for remicade also, contact your state insurance commissioner and ask them if there is an independent review you can apply for where actual medical professionals who aren't connected to the insurance company in any way can decide if humira or remicade are the right choice for you...and their decision is legally binding (at least that is how California does it). You have to take the time to talk to someone or email someone in the commissioners office however, and don't stop trying until you are able to physically talk to someone you feel will know the answer. Also, use the website to research what process you can go through to appeal a decision from both inside and outside the insurance company in your state.
If it is an HMO I don't really know as much about how those work, but my understanding is they do have more control over treatment. However, I still believe you are able to appeal their decision.
When you talk to the insurance company make sure you tell them you are reporting this incident to your state insurance commissioner and then follow through. It seems trivial, but this can in fact help get what you want because with enough reports like this, the company gets investigated. Then they find thousands of "mistakes" that they get fined for. They want to keep the attention of investigators away from them, so use your leverage if you can.