Boarders, just like private property, violate the NAP in the most fundamental ways. You simply cannot restrict movement on any land that you are not actively using and would disturb you.
It doesn't matter if that's a boarder with check-points, or if that's at your 500mile long ranch in the Arizona desert when you live in Texas.
It's fundamentally aggressive and to claim it as otherwise is simply dishonest and sickening, especially if you claim force is reasonable to protect such areas.
Private property as in personal property, such as the house you live in and the area around it, or anywhere else nearby you "own" that could have issue, such as if you have a farm with crops surrounding your house, isn't violating the NAP so long as you allow passage if need be, or for a leave-no-trace sleeping of the visitors if you own all the land around.
Otherwise you're the aggressor as you leave the other party no other option but to, in your opinion, violate your rights.
Europe and the USA will have literally a billion refugees because of how corporates and animal-eaters are ruining the environment, what they have is not something they need to deal with. They need to deal with the future, this problem they have now of some million is a non-issue.