1 Corinthians 5
As a church, it is our duty to discipline those within the church. God will handle those outside the church, but the reality is is if we don't help our members repent for the ongoing sin they are committing, their salvation is in question. Paul's letter to the church in Corinth explains the danger of allowing someone to continue in sin, and the process to which you should approach them to help them deal with it. And ultimately, if the person refuses to repent, Paul goes on to explaining how to expel that person from the church for their sin.