Some Christian groups practise various forms of shunning for former members. Some people who are members of Christian groups will shun even if their group does not explicitly teach it. I've seen books and handbooks that mention this passage as the reason for shunning:
As I have written to you in an epistle: "Do not associate with fornicators," certainly not with the fornicators of this world, nor with the greedy, nor with robbers, nor with the servants of idolatry. Otherwise, you ought to depart from this world. But now I have written to you: do not associate with anyone who is called a brother and yet is a fornicator, or greedy, or a servant of idolatry, or a slanderer, or inebriated, or a robber. With such a one as this, do not even take food. For what have I to do with judging those who are outside? But do not even you yourselves judge those who are inside? For those who are outside, God will judge. But send this evil person away from yourselves. [1 Corinthians 5:9-13]
What is your practise and what do you make of saint Paul's words in the passage?