Yes. Newspapers have a First Amendment right to refuse letters to the editor, as well as ads and submitted columns.
Since most newspapers are privately owned entities, whose editors have editorial control, they are free to promote whatever political, social or economic view they wish.
The U.S. Supreme Court addressed the issue of editorial control and freedom of the press in 1974 in the case Miami Herald Publishing Co. v. Tornillo, 418 U.S. 241.
This case concerned a Florida political candidate who brought suit against The Miami Herald pursuant to the state’s “right-to-reply” statute after the paper refused to print the candidate’s reply ...