Delete spam comments from every site on a WordPress network, slowly but surely

gistfile1.php

<?php

function qw_delete_spam_comments() {
	$in_progress = (bool) get_site_option( 'qw_delete_in_progress' );
 	
 	if ( ! $in_progress ) {
 		global $wpdb;
 
 		update_site_option( 'qw_delete_in_progress', '1' );
 
 		// 4980
 		$next = (int) get_site_option( 'qw_delete_next_blog' );
 		if ( empty( $next ) ) {
 			$next = 1;
 		}

 		if ( $next > 4980 ) {
 			return;
 		}
 
 		switch_to_blog( $next );

 		$spams = $wpdb->get_col( "SELECT comment_id FROM {$wpdb->comments} WHERE comment_approved = 'spam' LIMIT 10"     );
 
 		if ( empty( $spams ) ) {
			$next++;
			update_site_option( 'qw_delete_next_blog', $next );
 		} else {
 			foreach ( $spams as $spam ) {
 				wp_delete_comment( $spam, true ); 
 				$wpdb->query( $wpdb->prepare( "DELETE FROM {$wpdb->commentmeta} WHERE comment_id = %d", $spam ) );
 			}
 		}
 		
 		// reclaim disk space
 		$wpdb->query( "OPTIMIZE TABLE {$wpdb->comments}" );
 		$wpdb->query( "OPTIMIZE TABLE {$wpdb->commentmeta}" );
     
 		restore_current_blog();
     
 		delete_site_option( 'qw_delete_in_progress' );
 
 	}
 }
 register_shutdown_function( 'qw_delete_spam_comments' );