Believe it or not, it's "deletion by addition".
Open the Overdrive console with your Zune hooked up, select any title, and click TRANSFER.
Next, click NEXT
Click ADVANCED OPTIONS
Now coms the trick: Click BROWSE
Locate the file you want to delete and RIGHT-CLICK on it, then select DELETE.
-ICE-
30 GB White x 3
30 GB Brown x 1
Thinking about a 120GB Blue