How do I treat someone like a tax collector as is recommended in the Gospels?
The passage in question is this:
If your brother sins against you, go and tell him his fault, between you and him alone. If he listens to you, you have gained your brother. But if he does not listen, take one or two others along with you, that every word may be confirmed by the evidence of two or three witnesses. If he refuses to listen to them, tell it to the Church; and if he refuses to listen even to the Church, let him be to you as a Gentile and a tax collector. Truly, I say to you, whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven. Again I say to you, if two of you agree on earth about anything they ask, it will be done for them by my Father in heaven. For where two or three are gathered in my name, there am I in the midst of them (Matt. 18:15-20).
A tax collector at the time was someone who was not welcomed into the community because he was considered to be a collaborator with a foreign power (in this case, Rome). As the context of the passage makes clear, Jesus is giving this instruction to his apostles and it regards their authority in the Church. If someone does not listen to a successor of the apostles (a bishop), there should be a just process within the Church for dealing with the matter. But if the person ultimately won’t listen to the Church, he can be cut off from the Church by the Church.
In short, you need not worry about how to treat someone “like a tax collector” because you do not have the authority to exclude someone from the community of believers. Only the Church can do that.