<?php

    function my_process_main_footer() {

        global $ub_globalfootertext;

        if( is_main_site() ) {
            // We are on the main site
            remove_action( 'wp_footer', array( &$ub_globalfootertext, 'global_footer_content_output' ) );
        }

    }

    add_action( 'wp_footer', 'my_process_main_footer', 1 );

?>