As a member of the government, Trump cannot selectively choose who can or cannot view things that are the public duty of his office. He can't ban people from watching White House videos on Youtube and he can't block people on his public facing government account.
This does not mean that Twitter or Youtube cannot ban them from their site or control what they do on their site. It means the government cannot selectively enforce who can or cannot view their content. If Trump came over to your home and asked to hold a SOTU, you have the right to turn him away. However if you agreed to let him hold a SOTU and he decides to turn away your friends because he doesn't like them, that would be against the law.
This does not mean that Twitter or Youtube cannot ban them from their site or control what they do on their site. It means the government cannot selectively enforce who can or cannot view their content. If Trump came over to your home and asked to hold a SOTU, you have the right to turn him away. However if you agreed to let him hold a SOTU and he decides to turn away your friends because he doesn't like them, that would be against the law.