Index: wp-comments-post.php
===================================================================
--- wp-comments-post.php	(revision 20991)
+++ wp-comments-post.php	(working copy)
@@ -55,6 +55,8 @@
 // If the user is logged in
 $user = wp_get_current_user();
 if ( $user->exists() ) {
+	if ( is_multisite() && ! is_user_member_of_blog() )
+		wp_die( __('Sorry, you are not allowed to post comments on this site.') );
 	if ( empty( $user->display_name ) )
 		$user->display_name=$user->user_login;
 	$comment_author       = $wpdb->escape($user->display_name);