Maybe in style, postbit or postbit_legacy depending on which you use, find
Code:
<div class="username_container">
Under that is:
Code:
<vb:if condition="$post['userid']">
Change that to
Code:
<vb:if condition="$post['userid'] AND !$show['guest']">
That will have the effect of completely disabling the dropdown profile menu for guests
Caveat, have not tried this myself....