Originally retrieved from this thread March 27th 2010.
Gnome stores your “recent documents” in a file called
.recently-used.xbel. To disable the menu, we simply delete the file and then create a directory with the same name, effectively breaking Gnome’s ability to store anything in the file.
The fix is performed by executing the following commands at a terminal:
rm ~/.recently-used.xbel mkdir ~/.recently-used.xbel
Then go to Places > Recent Documents and select Clear Recent Documents.
The menu item should now grey out.