If you are wanting to remove the ability for people to add urls, there are a couple of things you'll need to do.
First, remove that BBCurl function mentioned above, and anything that relates to it, such as this URL button in the toolbar.
<input type="button" class="button2" accesskey="w" name="addbbcode16" value="URL" style="text-decoration: underline; width: 40px" onclick="BBCurl()" title="Insert URL: http://url or [URL text](http://url)" />
Then there are other parts of that page that relate to urls, such as in the bbtags array
var bbtags = new Array('[b]','[/b]','[i]','[/i]','[u]','[/u]','
and the helpline message
w: 'Insert URL: http://url or [URL text](http://url)',
and in the following message
is <em>ON</em><br />