Splitter!
Reasonable people can disagree with the following, but here is my view:
Libertarians should reach out, especially to the 25% or so of the population with some libertarian tendencies. Respect them, be open to learning from them, and try to present libertarian ideas, history, and solutions in a persuasive way. Many might move in a more libertarian direction, and in this way we can incrementally change society for the better. Accept that most will not go all the way and become extreme libertarians.
Barnett's views are lodged in a deeply libertarian part of the Nolan Chart. If the Nolan Chart asked about foreign policy, he would not be so deep into the quadrant, but he would not be off the reservation.
Rather than hunting down deviationists, we should be looking for allies.