He can stop them from using his account to make them.Nothing is preventing the user from saying whatever they want on their Twitter account ... Unless they screw up and Twitter bans them ...You're focusing on the first sentence of the First Amendment.
Read the last sentence - that's what this ruling is based on.
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
Nothing is stopping the ability of the user to redress their grievance ...
They can even do so on Twitter if they feel like it.
And it doesn't matter anyway ... Because Congress still didn't pass a law that prohibits the user from redressing their grievance.
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The First Amendment is fully incorporated. It applies to all branches of government, state and federal. It is not limited to the legislative branch.
Incorporation of the Bill of Rights - Wikipedia
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By blocking people on Twitter, Trump is denying them the right to respond to his official government pronouncements. He doesn't have to read those responses, but he can't stop people from making them.
You don't appear to understand how Twitter works.