From the admin module on IP Banning:
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From here you can Block individual IP Addresses or Ranges.
Anyone falling into a block IP address or range, will find many of the functions of the board disabled, including registering, and posting.
Be careful when blocking IP addresses as you may block legitimate users. Many people share the same IP address, like AOL users, blocking one may block another 500,000 users from using your board.
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I have not yet had to ban an IP using the forum, but I believe it only prevents forum use (ie registering, posting, etc). However, remember that unless the user has a static IP, then you would need to ban their ISP's range of IP's which could potentially prevent other users from accessing the board. Now, if you know this person has a static IP, then banning their IP may be a decent idea but it should always be used with discretion.
Personally, I'd say make the user account inactive and if they own their own domain (ie bob@hispage.com) then ban that domain if this person is a real problem, but again even if the person owns that domain then you could still potentially block other users.
You can't prevent someone from accessing the board, but you can make it more difficult.
Edited by xeerex