Creating single select WordPress taxonomies. Explanation at http://sudarmuthu.com/blog/creating-single-select-wordpress-taxonomies/

error_msg.php

<?php
/**
 * Display an error message at the top of the post edit screen explaining that ratings is required.
 *
 * Doing this prevents users from getting confused when their new posts aren't published, as we
 * require a valid rating custom taxonomy.
 *
 * @param WP_Post The current post object.
 */
function show_required_field_error_msg( $post ) {
	if ( 'movie' === get_post_type( $post ) && 'auto-draft' !== get_post_status( $post ) ) {
	    $rating = wp_get_object_terms( $post->ID, 'movie_rating', array( 'orderby' => 'term_id', 'order' => 'ASC' ) );
        if ( is_wp_error( $rating ) || empty( $rating ) ) {
			printf(
				'<div class="error below-h2"><p>%s</p></div>',
				esc_html__( 'Rating is mandatory for creating a new movie post' )
			);
		}
	}
}

// Unfortunately, 'admin_notices' puts this too high on the edit screen
add_action( 'edit_form_top', 'show_required_field_error_msg' );

meta_box.php

<?php
/**
 * Display Movie Rating meta box
 */
function movie_rating_meta_box( $post ) {
	$terms = get_terms( 'movie_rating', array( 'hide_empty' => false ) );

	$post  = get_post();
	$rating = wp_get_object_terms( $post->ID, 'movie_rating', array( 'orderby' => 'term_id', 'order' => 'ASC' ) );
	$name  = '';

    if ( ! is_wp_error( $rating ) ) {
    	if ( isset( $rating[0] ) && isset( $rating[0]->name ) ) {
			$name = $rating[0]->name;
	    }
    }

	foreach ( $terms as $term ) {
?>
		<label title='<?php esc_attr_e( $term->name ); ?>'>
		    <input type="radio" name="movie_rating" value="<?php esc_attr_e( $term->name ); ?>" <?php checked( $term->name, $name ); ?>>
			<span><?php esc_html_e( $term->name ); ?></span>
		</label><br>
<?php
    }
}

register_taxonomy.php

<?php
/**
 * Register 'Movie Rating' custom taxonomy.
 */
function register_movie_rating_taxonomy() {
	$args = array(
		'label'             => __( 'Rating' ),
		'hierarchical'      => false,
		'show_ui'           => true,
		'show_admin_column' => true,
		'meta_box_cb'       => 'movie_rating_meta_box',
	);

	register_taxonomy( 'movie_rating', 'movie', $args );
}
add_action( 'init', 'register_movie_rating_taxonomy' );

save_post.php

<?php
/**
 * Save the movie meta box results.
 *
 * @param int $post_id The ID of the post that's being saved.
 */
function save_movie_rating_meta_box( $post_id ) {
	if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
		return;
	}

	if ( ! isset( $_POST['movie_rating'] ) ) {
		return;
	}

	$rating = sanitize_text_field( $_POST['movie_rating'] );
	
	// A valid rating is required, so don't let this get published without one
	if ( empty( $rating ) ) {
		// unhook this function so it doesn't loop infinitely
		remove_action( 'save_post_movie', 'save_movie_rating_meta_box' );

		$postdata = array(
			'ID'          => $post_id,
			'post_status' => 'draft',
		);
		wp_update_post( $postdata );
	} else {
		$term = get_term_by( 'name', $rating, 'movie_rating' );
		if ( ! empty( $term ) && ! is_wp_error( $term ) ) {
			wp_set_object_terms( $post_id, $term->term_id, 'movie_rating', false );
		}
	}
}
add_action( 'save_post_movie', 'save_movie_rating_meta_box' );