Sure, IP banning would be a great idea, but it isn't very helpful in most cases.
Many trolls that get banned could be using dynamic IP addresses. Plus, there is the possiblilty that once the troll's IP address is released, a legitimate user could claim it and get barred from accessing or making posts-even though he did nothing wrong.
Example-A user named Troll1 is using a dynamic IP address (Troll1 also goes under names such as 'nutcase' 'listenbuddy' 'trollmaster', etc). That troll then gets banned based upon IP address.
Troll1's IP address then gets released. Then, another user (example "Goodboy2") gets that IP address. "Goodboy2" tries to access this site. He gets a message saying that he's barred, even though he did nothing wrong.
-ottermi619-
There you have it folks.