Howdy powerbook,
Yes it's possible with conditional templates
In the postbit template, find
Code:
<if condition="$post['signature']">
<!-- sig -->
<div>
__________________<br />
$post[signature]
</div>
<!-- / sig -->
</if>
and change it with
Code:
<if condition="!$show['guest']">
<if condition="$post['signature']">
<!-- sig -->
<div>
__________________<br />
$post[signature]
</div>
<!-- / sig -->
</if>
</if>
I haven't tried this out myself, but it should work!