Think of it this way: If a man owns a hundred sheep and one lamb wanders away and is lost, won’t he leave the ninety-nine grazing the hillside and thoroughly search for the one lost lamb?
And if he refuses to listen, then share the issue with the congregation in hopes of restoration. If he still refuses to respond, even to the church, then you must disregard him as though he were an outsider, on the same level as an unrepentant sinner.