What is the ruling on refusing to marry a Muslim to a non-Circassian, and boycotting whoever marries outside the Circassian community?

What you have mentioned about this group—their refusal to marry or allow marriage from outside their tribe—is wrong, unjust, and contrary to Islamic law. Superiority is based on piety, not lineage. The Prophet (peace be upon him) and his companions arranged marriages between people of different tribes and ethnicities. This restriction within the group leads to various harms, such as depriving women of marriage, or marrying them to those who are not righteous. It also contradicts the Prophet’s command to marry those who possess good character and religious devotion. Boycotting those who marry outside the group is an evil and an injustice. The obligation is to return to the ruling of Allah.