Banning a user based on IP address would be difficult, as many internet service providers use dynamic addresses, which means that you may not always have the same IP address everytime you log onto the internet.

Also, the use of email addresses is hard to police, as the creation of email accounts is too easy to accomplish.

The only way to ensure that an account is set to a single individual would be to link it to a credit card, and you can imagine how popular that would be with everyone. I certainly would not be a member here.