The idea behind the Internet Bad Neighborhood concept is that the probability of a host in behaving badly increases if its neighboring hosts (i.e., hosts within the same subnetwork) also behave badly. This idea, in turn, can be exploited to improve current Internet security solutions, since it provides an indirect approach to predict new sources of attacks (neighboring hosts of malicious ones).

via Internet bad neighborhoods – UTpublications.

This is a good start in developing some kind of IP blacklist.