Coercion != institutional coercion. Spontaneous self defense is an obvious example of coercion that isn't institutional, as in an established practice. If someone punches you in the face, you don't stop to think about what the established customs and practices dictate should be done, no, you punch back. A community kicking someone out because they can't behave, while it would probably end up becoming established practice in time, is not established in that sense before it has become a habit for the community. Humans form institutions all the time, that is unavoidable because problems will pop up that require solutions, and then applying those solutions will become habitual because that's more efficient than thinking every problem over from first principles. That's why anarchism is a never ending struggle with no end and the anarchists within that society would have to start raging against the exile-machine when it starts to exile people who shouldn't be exiled (like maybe people who disapprove of that institution). But setting out to build organized institutions of coercion even before a problem arises that would require a coercive solution that would afterwards become established practice, is backwards. The people who build this hypothetical society should cross that bridge when they get there and hopefully enough of them will realize to constantly keep a sceptical eye on any of those practices so that they don't become too established so as to cause unnecessary harm by sheer inertia.