Do what you feel you must, but keep this in mind:
"Now I urge you, brethren, note those who cause divisions and offenses, contrary to the doctrine which you learned, and avoid them. For those who are such do not serve our Lord Jesus Christ, but their own belly, and by smooth words and flattering speech deceive the hearts of the simple."
- Romans 16:17-18
"But now I have written to you not to keep company with anyone named a brother, who is sexually immoral, or covetous, or an idolater, or a reviler, or a drunkard, or an extortionernot even to eat with such a person.
For what have I to do with judging those also who are outside? Do you not judge those who are inside? But those who are outside God judges. Therefore put away from yourselves the evil person."
- 1 Corinthians 5:11-13
"If anyone teaches otherwise and does not consent to wholesome words, even the words of our Lord Jesus Christ, and to the doctrine which accords with godliness, he is proud, knowing nothing, but is obsessed with disputes and arguments over words, from which come envy, strife, reviling, evil suspicions, useless wranglings of men of corrupt minds and destitute of the truth, who suppose that godliness is a means of gain. From such withdraw yourself."
- 1 Timothy 6:3-5
Nathan is not a big deal. Treating him as such only serves to play into his hands, and validates him to himself and to those whom he influences. He has already cut himself off. The proper response is clear, as Saint Paul instructs us above.
One last quote, this one from a heiromonk, and specifically about Nathan:
"Father N is known to me and is a WR recent convert and quite full of himself and he and his wife are all ways superior to the rest of us sundry mortals. I have had a few rounds with him in his non-Orthodox times. I do not see him as anything but a distraction. He and his disrespectful wife like to be indignant on a regular basis because they know everything. They will add nothing to the conversation but for egos sake dominate it. Disregard them for the good of the cause!"
But if you really want to be dragged into his sphere of influence, go ahead, and let him goad you into a "response". That's really all he wants, and you are playing right into his egotistical hands.
Unless something changes, this is the last time I will even reference him. As far as I am concerned, he is anathema.