Here are the instructions to achieve it:
Create a new banner wrapper at Configuration > Banners&Links Format and into that new wrapper add:
<script type="text/javascript">document.write("<iframe name='banner' src='{$htmlcompleteurl}' framespacing='0' frameborder='no' scrolling='no' width='{$width}' height='{$height}' allowtransparency='true'><a href='{$clickurl}' rel='nofollow' target='_top'>{$bannername}</a></iframe>");
</script>
<noscript>
<h2><a href="{$targeturl}">{$bannername}</a></h2>
</noscript>
When creating/editing your banners make sure you choose the freshly created wrapper from the 'Select Wrapper For Banner' drop-down menu.
In the case of simple Link Banners you can create a wrapper with the following code:
In the case of simple Link Banners you can create a wrapper with the following code:
<a href="{$clickurl}" rel="nofollow" target="_top">{$clickurl}</a>