Well you can't do it hakf-way, allow homepages, but stop other sites. The easiest was I can think of blocking links in signatures is to use CSS. If that sounds good say so and I'll look up the exact code you will need.
That one is about blocking images in signatures ; you can do the same for URL's I guess, by removing the fString = ReplaceURLs(fString) line from the formatsig function.
Simple (I think ) in the code, find where you are rendering the sig. At that point scan for <a and if present scan for </a> then remove the tags, and everything in between.