That's easily got round. For example, if I were a bit nastier than I am, I could use a dial- up modem, getting a different IP address each time I connect, perhaps using a variety of pay-by-the-minute ISPs, and post any sort of abuse or provocation. Or I could post from a library, or an internet cafe etc. etc. etc.
If you really want to be able to police posts, you should restrict posts to members, or at least insist on a valid email address for guests as many blogs do.
I suspect it's possibly the will that's lacking, both for the laudable reason that the good atmosphere comes in part from openness, and perhaps for the slightly less praiseworthy reason that some regulars can abuse others anonymously when the spleen takes them.