By your own admission, a passenger tried to board out of order. The passenger was caught, told not to board, and made some kind of a fuss.
Whatever the fuss, it resulted in the passenger being offloaded and eventually permitted to board.
What is missing from this narrative is exactly what the passenger said and did when caught. Although you have not said so, it sounds as though the "passenger" was either you or your companion or you somehow witnesses all of this.
Whether offloading the passenger was an overreaction is entirely dependent on exactly who said what and when and you have not supplied that detail. WN does not offload passengers lightly, so without detail, one has to presume that the fuss was sufficient.
Not sure how we get to "shackled and gagged". When people use hyperbole such as that without supporting facts, it tends to cast a lot of doubt on their story.