First of all, open up a terminal window for commands:

1.) To clear recent documents, use:

rm ~/.local/share/zeitgeist/activity.sqlite
zeitgeist-daemon --replace

2.) To prevent new items from being created, use:

echo -n > ~/.recently-used.xbel
sudo chattr +i .recently-used.xbel

If you change your mind, use this to restore:

sudo chattr -i .recently-used.xbel

Thanks to multifo from ubuntuforums.org.