To some modern minds these instructions seem unduly rigid and harsh. A great part of the problem, however, lies in the modern inclination to be highly tolerant of religious differences. One must frankly face the fact that the New Testament writers did not share this spirit of toleration. Their commitment to the truth and their consciousness of the dangers of religious error called forth many stern denunciations of false teachers. Not surprisingly, this modern age, having a diminishing sense of the dangers of heresy, has lost its convictions about the truth.
But the passage ought not to be taken beyond the writer's intent. He was thinking about false teachers actively engaged in disseminating error. In this activity they are not to be helped at all. Even a word of greeting might tend to give them a sense of acceptance that could be misconstrued. The readers were to make plain from their aloofness that they in no way condoned the activities of these men. The same must be true today.
Zane C. Hodges, "2 John" in Bible Knowledge Commentary: New Testament, (USA: SP Publications, 1983), 908.