Let's take a css approach. Please edit your macro:
<override_macro name="message_with_signature" requires="node">
<n.node_message_as_html />
<n.if.both condition1="[n.owner.has_signature/]" condition2="[n.owner.is_active/]">
<then>
<div class="signature weak-color signature-[n.owner.user_email/]">
<n.owner.signature_as_html/>
</div>
</then>
</n.if.both>
</override_macro>
In this way, every signature's html div will have a class named
signature-user_email.
Finally, add the css rule in your css code which shows off the signature for the specific user. Example:
div.signature-pedroabg@gmail.com {
display:none;
}
It will hide my signature.