Quick code edit
 

<vb:if condition="$post['isfirstshown']">
<li class="firstpost_advert_container"><div class="firstpost_advert">
<script type="text/javascript"><!--
google_ad_client = "ca-pub-xxxxxxxxxxxxxxxx";
/* 728x90, created 8/7/09 */
google_ad_slot = "xxxxxxxxxxxxxxxx";
google_ad_width = 728;
google_ad_height = 90;
//-->
</script>
<script type="text/javascript"
src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
</script>
</div>
</li>

Can someone help me modify this code so that I can input a forumid to prevent the ad from showing up in that section?

--------------- Added [DATE]1347910676[/DATE] at [TIME]1347910676[/TIME] ---------------

Anyone

<vb:if condition="!is_member_of($bbuserinfo, 1,2,7)">

This shows your ad to members of usergroup 1, 2 and 7.

Edit.
Never mind, wrong read, my bad.
Thought you ment usergroup.

What you want is this:
<vb:if condition="$forum[forumid] != x">Show this if forum id is not x</vb:if>