function process_tag_removals( $tags, $user_id ) {

    foreach( $tags as $i => $tag ) {

        if( strpos($tag, 'REMOVE_') !== false ) {

            $remove = str_replace('REMOVE_', '', $tag);
            wp_fusion()->user->remove_tags( array( $remove ) );
            unset( $tags[$i] );

        }

    }

    return $tags;

}

add_filter( 'wpf_apply_tags', 'process_tag_removals', 10, 2 );