> when used by someone without a firm grasp on reality
Implicit in this question is the idea that everyone viewing the thread is on the same page about why the person is spewing racist crap. This is never the case.
Imagine you stumble upon a brand new forum, where you know nothing about the regulars or years of built-up social dynamics. You find someone making racist remarks, and everyone is just... fine with it. You have finite time before you die: you're not going to wait around to see if there's a good reason for it. The reasonable thing to do is write it off as a forum that tolerates racism and move on with your life.
What do you want to do, flair people with "kind of crazy, don't worry if they say racist shit sometimes" labels? How are you planning to verify these people and tell them apart from people who just want to say racist shit? I guarantee there are a lot more of those than there are Terry Davises. This is, to put it gently, not scalable.
I've seen this is at small scale, too. Tolerance leads to the problem getting worse over time. You need to stand up for your rules of conduct, and hope the offender learns the right lesson from it. If not, that's not your responsibility. Your responsibility, as a group leader or admin, is the environment everyone shares.