Who told you that? The word "open" is one of the most overused words in American marketing, and has never been an implicit grant of rights to anyone. People were routinely banned from project mailing lists long before codes of conduct became common, and corporate projects labeled "open" were often completely closed.
I think that require to leave legally refers to situation where that person comes back again and again and again. So that you may use your lawyer to threaten him or his layer.
# Code of Conduct
in the United States, the development of the Code of Conduct appears to be a mechanism, refined by lawyers, to make a legal case to -> kick someone out <- of an 'open' system
The code of conduct uses words to describe desired outcomes of cooperation, but actually, it defines boundaries such that there is a specific reason to require a person with some behavior pattern, to leave, be blocked and removed from the project.
Legally, if the project is declared 'open' then there was no way to require someone to leave, legally. IANAL