I'd say do what you will, and if you reject their code, they'll probably be fine living on in a fork of your project. Personally, I wouldn't mind using code from someone I disagreed with, because in the end, our common interest in a piece of software might be one thing that brings two opposing parties together, rather than divides them apart.
> What to do with a contributor to your open-source project who turns out to be a very bad person—is worth contrasting with what would unfold were you his manager at a firm: You'd fire the person.
You'd better be damn careful about that. It's illegal in New York, for example, to fire someone for their (legal) associations outside of work, when not representing the company. If they say something objectionable to a coworker, you're probably fine. But you can't fire someone for being a member of an objectionable group or taking part in a protest or anything remotely like that unless they somehow represent the company while engaging in the objectionable activity.