Most ISPs provide their customers with dynamic IP addresses, which are allocated whenever a user connects to the internet. To change their IP address, all that someone needs to do is disconnect from the internet and then reconnect.
Even if someone has a static IP address, there are plenty of free proxy servers available (either directly or using the Tor project) which can be used to mask that address.
So any registration or banning system, which is dependent upon IP address recognition, simply won't work.
Chris