You can do this in the template, using a trick that creates a value that lives in PHP. Presuming you use the regular postbit to display posts, as opposed to the legacy postbit, then look in in the "postbit" template for these lines: <if condition="$post['signature']">
<!-- sig -->
<div>
__________________<br />
$post[signature]
</div>
<!-- / sig -->
</if>The line you want to replace is the first one, the "<if ... >" test.
Try replacing it with this:
<if condition="$post['signature'] AND !$saw_signature_for[$post[userid]] AND $saw_signature_for[$post[userid]]=1">
In plain english, what this test means is:
if there is a user signature for this post
AND if I have not yet seen it
THEN note I have seen it and output the HTML for it
For the same page, the next time that a post is seen from this user, the second test will fail and the signature will not be displayed.