The Geneva Conventions require that any such person be afforded due process. Due process in such instance is defined, by the common law of nations and by U.S. common law, statute, and case law as a military tribunal.
Well, if you have any experience with the Executive Branch, you'll know that 'due process' is any official process: obviously, in this case, the due process is to be shot out of hand.
Actually, I'm not so much in favor of shooting them as I am of tying them to stakes, heaping rolled copies of the Constitution around their feet and setting fire to the whole sheebang.