Interesting... I was able to do as you say, edit the XML files, make them Deleteable="true", and it seemed to work from within QXP.
But when I rebooted, they (all of the default print and PDF styles) were back again. Weirdly, the folder in my Prefs folder only had the four XML files I wanted to keep.
The Publi Script Forum
They taught me everything
http://publi-script.com/Forums/index.php?board=4.0