https://www.techdirt.com/articles/20190109/14451841365/appeals-court-first-amendment-violation-to-ban-members-public-govt-officials-facebook-pages.shtml

Late last spring, a federal court in New York made it clear Trump's blocking of Twitter users violated those users' First Amendment rights. As the court reasoned then, Twitter may be a private company, but the use of it by government official to engage with the public makes it a limited public forum -- limited to Trump's account and Twitter users' interaction with it. Twitter remains free to moderate as it pleases. The ruling did not say Twitter itself was a public forum, just government officials' use of the platform via official accounts.

When Trump's account blocked people he didn't like, he violated the First Amendment.