I managed to figure out how to clear and permanently disable this in Expose so figured I'd post here.
I'm using BBEdit but imagine the steps are the same for TextEdit(?)
You have to set the Recent Items to 0 (as above) AND delete the contents of:
/Users/[your username here]/Library/Preferences/com.barebones.bbedit.PreferenceData/
(As I say I imagine there's something similar for TextEdit)
This folder rather astonishingly contained the history of every file I have EVER edited in BBEdit(!!) some 4000 odd files - also a security risk in my book.
If you were very paranoid (as am I) you could also chmod this folder to be readable only so it is never written to:
sudo chmod -R 444 /Users/[your username here]/Library/Preferences/com.barebones.bbedit.PreferenceData/
Hope that helps... Apple/BBEdit should really include this functionality within Expose as an option so we can turn it off in my humble opinion.