function disallow_insert_term($term, $taxonomy) {

    $user = wp_get_current_user();

    error_log("{$term, $taxonomy}"); //inspect term and taxonomy
    error_log(implode(', ', $user->roles); //inspect the user roles

    if ( $taxonomy === 'post_tag' && in_array('somerole', $user->roles) ) {

        error_log('yes, disallow this term...'); //do we get into the conditional

        return new WP_Error(
            'disallow_insert_term',
            __('Your role does not have permission to add terms to this taxonomy')
        );

    }

}

add_action( 'pre_insert_term', 'disallow_insert_term', 10, 2);