Thank you sirryche! I patched the tpl_footer.php and everything works fine! No more spider sessions spamming the user tracking page :).

I did it not exactly the way you described. Here is what I did:

/catalog/includes/templates/template_default/common/tpl_footer.php
(on install, I did not had the tpl_footer in the templates/<MYTEMPLATE>/common directory, so I used the common one)

Code:
<?php if (ZEN_CONFIG_USER_TRACKING == 'true' && !$spider_flag) { zen_update_user_tracking(); } ?>