We instruct you, brothers,
in the name of (our) Lord Jesus Christ,
to shun any brother
who conducts himself in a disorderly way
and not according to the tradition they received from us.
Also in this same chapter (verses 14 and 15), Saint Paul says:
If anyone does not obey our word
as expressed in this letter,
take note of this person not to associate with him,
that he may be put to shame.
Do not regard him as an enemy
but admonish him as a brother.
Saint Paul is speaking here specifically of people who have stopped working or taking any prudent actions because they think our Lord’s return in imminent.
In this case, Saint Paul advocates shunning, partially so that his readers do not follow along with that activity and partially as a symbolic act in order to shame the person (and encourage them to return to good belief and practice).
In these days when prominent Catholics not only publicly support intrinsically evil acts but actually argue with the successors of the Apostles about Catholic teaching and tradition, it is easy for people to reach for the “shun button” and shout “anathema sit!”
Penalties such as excommunication and interdict, of course, are to be imposed only in accordance with Church law and only by the bishops entrusted with that responsibility. We must also remember that such penalties, like the shunning of which Saint Paul writes, are intended to help bring the offender to repentance.
In our own lives, in dealing with those who fall from the faith to some extent, we should exercise prudence and charity and above all we should pray, for ourselves and for them.