Maybe this:
X, Y, Z are the groups which are not supposed to see the ads.
<if !array_intersect(array('X', 'Y', 'Z'), explode($mybb->user['additionalgroups'] ? $mybb->user['usergroup'].','.$mybb->user['additionalgroups'] : $mybb->user['usergroup'])) then>
<script src="http://ads33751.hotwords.com.br/show.jsp?id=33751&cor=005F00"></script>
</if>
X, Y, Z are the groups which are not supposed to see the ads.