It would be impossible to stop him completely, but you can make it much harder for him.
Cookies
You can attept to store a cookie on the banned users computer. As long as the user doesn’t delete his cookies or change browser, you can ban his new ips.
Registration
You can require new users to register and activate account their account via email. This will slow any banned users down a lot as for every account they make, they’ll need a new email.
Automatic ban
You could add a ban button for yourself, to make it easy to ban people.
You could add a vote ban feature so other members could ban people temporarily.
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solved Ways of banning someone from a website [closed]