Adding scripts in forms (safely)

[V]

This might seem like a bad idea but.. which would be the safest way to allow website visitors to paste scripts into a form? Generally affiliate links like

 

<script type="text/javascript" src="http://www.hostmonster.com/src/js/someuser/CODE81/88x31/hm_88x31_01.gif"></script>

 

or Adsense

 

 

<script type="text/javascript"><!--
google_ad_client = "pub-0000000000000000";
google_alternate_color = "ffffff";
google_ad_width = 468;
google_ad_height = 60;
google_ad_format = "468x60_as";
google_ad_type = "text_image";
google_ad_channel ="0000000000";
//--></script>
<script type="text/javascript"
  src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
</script>