The userid is 0 if the user is not logged in. You could also check for the groupid but it should give the same result.
It works for me, so maybe I don't understand what you want. If you want the link to be inactive, then edit the postbit or postbit_legacy template and put
Code:
<vb:if condition="$show[member]"> ...</vb:if>
around the link. (The reason I chose the other method is that, while taking out the link will stop them from clicking on it, it won't keep guests from going directly to the profile url).