There is not currently an "automatic" way to delete the URL list. However, it you open up the prefs.js file in your profile directory located here:
Profiles\<profile_name>\*.slt\prefs.js
and look for entries like this:
kmeleon.MRU.URL#
Delete those entries. You can also control the total number of entries saved their by changing this number:
kmeleon.MRU.maxURLs
Make sure to do this with the browser closed. For more info. on customizing prefs, see:
http://kmeleon.sourceforge.net/docs/custompref.php
Andrew