I've seen this question asked many times so here's how to clear the Zeitgeist history. To reset the Zeitgeist history, open a terminal and copy/paste the following commands:
rm ~/.local/share/zeitgeist/activity.sqlite
zeitgeist-daemon --replaceThat will only clear the history, but will not stop Zeitgeist (so the files you access after running the above commands will still be tracked by Zeitgeist).
