William Burkitt on 2 John 1:10

In these words our apostle directs them how to carry themselves towards those wicked apostates and heretical seducers, that deny the gospel, or any essential part of Christianity; by no means to give any countenance or encouragement to them, or to hold any familiarity or communion with them, nor to entertain them, nor so much as courteously to salute them, but, by showing them disrespect, manifest a dislike and disapproving of their errors. This form of interdict, Bid him not God speed, seems to be an imitation of the Jewish practice towards excommunicated persons, who were not only excluded from all commerce, but also from all kind of common civilities, and ordinary salutation.

Learn hence, That even civil courtesy, and common respect, is not, ought not, to be paid to those that seduce others, or attempt to seduce us, from the Christian faith.