Stupid decision. President should defy it.
The First Amendment bars Congress from making a law that abridges freedom of speech. You could extend that to executive actions that involve authority granted by Congress. But Twitter is a private account for which the President's usage is no different than that of any other citizen. There is no shred of government action/authority involved in blocking a user -- it is purely a right being exercised by the President himself that is open to any other citizen.
If it was a government account, rather than his personal account, that might be different. But it isn't.