Here are the sentences with the underlined phrases replaced by words with the specified prefixes:
Sue's become strange; she is avoiding the company of other people.
- association
Nancy is still on good terms with the man who used to be her husband.
- ex-husband
His attitude showed that he was against everything related to the United Nations.
- anti (for "everything related") / anti-UN
Bill is such a good helper; he is willing to work in conjunction with others.
- cooperation
In this country, the society is made of people of diverse origins and backgrounds.
- multicultural society
This school accepts students from Europe, Asia, and Africa.
- intercontinental
They were doing research on the trades that existed here before the colonization.
- pre-colonial