I have been having 300+ users on the site recently and for a small site with little more than 150 users and rarely more than 10 on at a time I installed IPTracker as I smelt something fishy.
In the first couple of minutes, I got the following:
IPT just tracks - you can exclude that info from being tracked but that's all. The zero IP may be the result of a small binary escaping issue that has since been addressed in DF 9.2.0.RC2, though I haven't updated IPT for that DF version yet.
If you wish to ban, you need to use the DF security system or your htaccess file - such matters are beyond the scope of these forums.