In Matthew 18, what does it mean to treat someone like a pagan and a tax collector?

The context deals with your responsibility to teach someone the right way. We all have that responsibility, but if a person keeps ignoring you and, worse, goes above you and does something even more wrong, then, as Jesus says in Matthew 7, do not cast your pearls before swine, because they will turn and rend you. Jesus is teaching us to be very judicious about whom we approach with the commandments or the gospel. When the apostles went to places where people said, we do not want you and your gospel, get out, Jesus did not tell them to go back and keep trying; he said shake the dust off your feet and go find someone who wants to listen. So you are not a slave who must keep preaching to people who beat you and refuse Jesus. We look for those who are interested, and we can usually tell who is and who is not. If someone clearly does not want Jesus, there is no reason to get into an argument; just be a good Christian and move on. We should never use that as an excuse to avoid evangelizing, but when someone has plainly made up their mind, Jesus is telling us not to force it.