It's clearly speech. Were the content of their speech different, there would be no ban. Or is there some means of behaving where they could relay that message to their governmental twitter account without a block?
At the extreme end of the spectrum, if every hour I repeatedly posted to Barack Obama's twitter: "NI---R NI---R NI---R NI---R NI---R NI---R NI---R NI---R NI---R NI---R NI---R NI---R NI---R" over and over, would that be speech or misbehavior? At some point, online conduct crosses the line from criticism (speech) to disruption/DOS (behavior).
> At some point, online conduct crosses the line from criticism (speech) to disruption/DOS (behavior).
Then it's up to the social media company to decide what to do about it. Twitter has a First Amendment of association. Twitter can ban the comments, or leave them up, or do anything in between. If Barack Obama were currently a government official, Obama would have latitude to ask Twitter to restrict N-word comments on his posts, but the First Amendment would prohibit him from coercing Twitter to restrict N-word comments, and would also prohibit him from blocking the commenters.
Note: Twitter has a First Amendment right to ban speech, not only conduct. "Has a right" doesn't mean "should", but in the first place the featured article is about a legal issue, not an ethical issue.