Just because I just did the same thing, some FYI. After cutting out the spiders lines from the /templates/default/island_whos_online.tpl as Sirdude mentions, the language entries to edit are in the file portal_islands.php in /languages/english and change ONLINE_ANON and ONLINE_ANONS to remove the "and" where it says "Guests and". Otherwise your visitors will be left with "10 Guests and" if you only chop the spider ref out of the template.
Warning that this will have the same effect anywhere that var is used, but I don't see it anywhere else.
