Maybe you should study beyond your civics class. Stated opinions of one individual against another individual are protected speech, and are not subject to defamation in civil lawsuits.
Milkovich v. Lorain Journal Co ., 497 U.S. 1 (1990)
" The U.S. Supreme Court has said that a statement is an opinion that merits protection when it is (1) about a matter of public concern, (2) expressed in a way that makes it hard to prove whether it is true or false, and (3) can’t be reasonably interpreted to be a factual statement about someone."
A few not-really-related concepts are being bounced back and forth there.
Is the charge here hate speech? No, it was harassment, which is conduct that doesn’t rely on the content of what you’re saying. But sometimes the content of what you say indicates a particular intent, which can be an aggravating factor. Burning a big pile of trash in someone’s yard at night is a crime, and burning a cross in their yard at night is also a crime. The latter would get you an elevated charge. Doing [crime] to a person because they are [insert category] is treated differently from saying you don’t like them because they are [insert category].
Does the First Amendment only mean that the government cannot make it illegal to criticize the government? No. If the government makes a law about what can be said in a conversation between private individuals, that is infringing on those people’s right to free expression. It’s equally true that the First Amendment means that the government can’t make it illegal to criticize chickens, or Kellogg’s, or Pete Sampras.
Is the Pit protected speech? Yes and no. If the government says it’s illegal to criticize Jeff Bezos or whatever, and people start getting arrested for what they post in the Pit, then yes, the First Amendment protects them from that. What it does not protect them from is consequences from other private individuals, i.e. speech in the Pit is not protected against a moderator telling you you’re not allowed to criticize them in the Pit.
Are opinions about other people protected speech? Yes. But they’re protected only in the same way that all other statements that don’t fall into some specific category are protected; opinions in and of themselves are not a specific special category. There has to be a compelling reason why it IS okay to sue someone for what they’ve said, and the general rule is you can’t. That Supreme Court case is about one of those specific reasons where you can sue somebody–defamation–and where the limits are.
No. As the SDMB is a privately owned operation the administrators and mods can shut down any discussion thread they desire for any reason they desire. With the usual caveat for protected classes. ie, shutting down / banning all black posters.