Code:
<if condition="$bbuserinfo['usergroupid'] != X">Code to display ads</if>
change X to the usergroup id of the group that dosn't display ads you can add multiple groups by doing:
Code:
<if condition="$bbuserinfo['usergroupid'] != X && $bbuserinfo['usergroupid'] != Y">Code to display ads</if>
you can also specify users not to display ads to as well by using $bbuserinfo['userid'] != X