I'm not the GP commenter, but I am. There is actually a so-called "wrong speech" or whatever you want to call it. Repressive speech such as open racism - we don't need that shit. Ever. And without it, we may cultivate actual free speech.
Also, you make conclusions based on bias. You can be racist and also violence free and vice versa.
Actually, I didn't assume they would be violent.
I'm not trying to be facetious or obtuse, I actually don't see how this could work. Speech is a natural right, i.e., I can speak (or not) *any* thoughts irrespective of the presence or permission of government (albeit not without consequences post-hoc). Any attempt to restrict that right proactively must (at least) border on tyrannical, and that's before we even get into how it would be decided which speech is acceptable and which must be censored. After all, even the example subject we're building this conversation around has at least two extant, incompatible definitions.
Yeah there's actually a super simple answer: it's not, and you don't. Why are you assuming I want to go into their homes to police what they say in private?
The first question (slightly paraphrased) was:
How do you prevent [the natural right of free speech] from overriding your efforts to restrict [speech in public fora] (word of mouth being the best advertising)?
Maybe I'm barking up the wrong tree. Is your intent not to limit the dissemination of "bad" (to be as broad as possible) ideas?