Excluding a Usergroup From Seeing Ads

I have a certain usergroup that I am wanting to hide advertisements from and I can't seem to find the code to do this (I see where to show it to certain usergroups but not exclude it)

My ad code looks like this:

<xen:if is="{$forum.node_id} == 16">
     <a href="some link""><img src="some banner ad"></a>

  <xen:elseif is="{$forum.node_id} == 26" />
     <a href="some link"><img src="some banner ad"></a>

  <xen:elseif is="{$forum.node_id} == 27" />
     <a href="some link"><img src="some banner ad"></a>

  <xen:else />
     <script>Google Adsense.......

What, and where, would I place some code to exclude usergroup 18?

I'm sure this has been covered before but I have looked everywhere for it (except where it is). Any help would be greatly appreciated.