“If your brother or sister sins, go and point out their fault, just between the two of you. If they listen to you, you have won them over. But if they will not listen, take one or two others along, so that ‘every matter may be established by the testimony of two or three witnesses.’ If they still refuse to listen, tell it to the church; and if they refuse to listen even to the church, treat them as you would a pagan or a tax collector. (Matt 18:15-17, NIV)
Most of what is said in the video above is very good. We must be careful to first ask the person who is complaining about someone if they have talked with that person about this issue. If they have, gently tell them that you're willing to listen to what they have to say about their brother or sister, but only while that brother or sister is present.*
Following Jesus guidelines on this issue is one of the most important things we can do. It is particularly important for churches. No one wants to be in a place full of gossips.
Don't be fooled by the title of the following video. This is the best lecture I've ever heard about how Christians, as individuals and as a church, should deal with a sinning brother.
"If they still refuse to listen, tell it to the church; and if they refuse to listen even to the church, treat them as you would a pagan or a tax collector." (Matt 18:17)
The Church is supposed to be a place full of warm, considerate and kind people. If a church is, then being kicked out ought to be enough to make a person think hard about what they've done and hopefully, bring them to their senses (see 1 Cor 5:5).
I wrote to you in my letter not to associate with sexually immoral people— not at all meaning the people of this world who are immoral, or the greedy and swindlers, or idolaters. In that case you would have to leave this world. But now I am writing to you that you must not associate with anyone who claims to be a brother or sister but is sexually immoral or greedy, an idolater or slanderer, a drunkard or swindler. Do not even eat with such people.
What business is it of mine to judge those outside the church? Are you not to judge those inside? God will judge those outside. “Expel the wicked person from among you” (1 Cor 5:9-13).