以下是我的想法，似乎奏效了：

add_filter( \'pre_post_tags_input\', \'no_tags_input_create\' );
add_filter( \'pre_post_tax_input\', \'no_tax_input_create\' );

function no_tags_input_create($tags_input) {

    $output = array();

    foreach( $tags_input as $tag )
        if( term_exists( $tag, \'post_tag\') )
            $output[] = $tag;

    return $output;
}

function no_tax_input_create($tax_input) {

    if( !isset($tax_input[\'post_tag\']) )
        return $tax_input;

    $output = array();
    $tags = explode(\',\', $tax_input[\'post_tag\']);

    foreach( $tags as $tag )
        if( term_exists( $tag, \'post_tag\') )
            $output[] = $tag;

    $tax_input[\'post_tag\'] = implode(\',\',$output);

    return $tax_input;
}