Just my 2 cents,
but free speech technically doesn't apply to online forums, reason being that the forum is not a government linked or affiliated entity. Because of this, the decision of the forum admin has to be objective towards the wellbeing of the board and community as a whole, not the protection of the users' rights.
My suggestion is to move the topic to a private forum, then thrash out the issue with the threadstarter via pm or whatever means applicable.
As what the others have said before, your status as forum admin gives you the right of the final say. Hope this turns out well for you