Oh, right, Sorry. Only allowing *logged in users* to see.
The online users shows things to guests too.
In moodle/blocks/block_olineusers.php after the lines in red, you can try adding the line in blue (backup and test on a non-production server first).
$this->content = new stdClass;
$this->content->text = '';
$this->content->footer = '';
if(isguest()) { return $this->content; }
Tim