Jesus warns the seventy-two that as they preach the kingdom, they will face opposition and rejection. When this happens, they are too wipe the dust off their feet as a sign and warning to them. The warning is that the kingdom of God has indeed come and as they reject the messengers (and therefore the message), so too God will reject them just as a person wipes dust off their feet. It is a case of complete rejection and not wanting to have even an expression of their lingering presence. It is a terrible thing to reject Christ.
This is evident in what Jesus continues to say. Jesus compares Galilean towns that had experienced his ministry to Sodom, Tyre and Babylon (the city desiring to be exalted to heaven). These were the classic immoral, idolatrous and sinful cities in the Old Testament. The greater one’s exposure to who Jesus is, the greater the judgement that comes in rejecting him.
As Babylon and Capernaum believed that their respective religious systems would exalt them to the highest point in the heavens, the actuality will be that their rejection of Christ will bring them to the lowest depths of Hades. It is a terrible thing to reject Christ.