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It technically could be considered an act of insubordination to violate a direct order from a supervisor. Now, that being said, it is more commonly considered simply a violation of policy rather than insubordination, which is more often connected with some verbally abusive response to a supervisor.
You can write it up as insubordination and that wouldn't create any sort of legal insufficiency, but this really is about the policy which is being violated. I would note that he has received notification of that policy and remedial training concerning the policy (any subsequent talks you've had with him about the policy), yet he still chooses to violate it.
I'm sorry. I left on family vacation and I'm just seeing your response. I suppose, I'm not sure what your question really means here. Why do you feel you have to state this as insubordination rather than simply leaving it as a violation of company policy? One has no greater power to allow disciplinary action than the other.