Anarchist societies will perhaps inevitably find themselves at odds with individuals who cannot or will not either find a place within the society or disassociate and distance themselves from it. They will introduce a non-anarchist element into relations and anarchists will have to find the means to balance their anarchist ideals and the actions necessary to preserve the anarchic relations that are threatened.

It is unlikely that the relations between anarchists and those who aggressively refuse anarchistic relations can themselves be anarchistic. They will instead by essentially warlike. The challenge for anarchists will be to respond to threats without being forced to reconstitute authoritarian institutions. That means that in the most extreme cases, anarchists may have to meet threats with violence, while not being able to avail themselves of the usually justifying and legitimating mechanisms of law and order.

Proudhon's writings on "moral sanction" are a good example of an anarchist response to the problem. He argued in his mature writings that society had no right to punish, but at the same time he defended the use of even lethal force by individuals in those extreme cases where no other solution could be found. However, he defended it as an unfortunate necessity, with potential consequences and no general moral sanction, not as an act that could be considered "justified" or "legitimate" in some a priori sense.

I think that it's hard for us to even consider what might or might not be necessary outside any sort of theory of "legitimate authority," but anarchists have at least been trying to imagine that outside since the beginnings of the tradition.