Recently a federal judge ruled Trump can’t block people on twitter, so how can twitter ban accounts without violating the 1st amendment directly?

Because there’s no contradiction. If Trump was a private citizen, he could block anyone he wanted to. He could resign tomorrow and block people to his heart’s content.

And Twitter can ban anyone they want, for any reason. Their site, their servers. The First Amendment is not an issue here at all. 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. It lays out what the government cannot do, nothing more.