Prevent tag creation on the post edition page

This will disable the Tags suggesstion:

function wpse207107_dequeue_suggest() {
    global $post_type;

    if ( is_admin() && $post_type == 'post' ) {
        wp_dequeue_script( 'suggest' );
        wp_deregister_script( 'suggest' );
    }
}
add_action( 'admin_enqueue_scripts', 'wpse207107_dequeue_suggest', 5 );

However, that might disable some other suggest functionalities if other plugins uses the script. Alternatively, you can entirely remove the Tags metag box for non-admin if you want them not to tag the post while keeping it for admins.

function wpse207107_remove_tags_meta_box() {
    if ( !current_user_can( 'administrator' ) ) {
        remove_meta_box( 'tagsdiv-post_tag', 'post', 'side' );
    }
}
add_action( 'do_meta_boxes', 'wpse207107_remove_tags_meta_box' );