The problem with phpBB is that it's free and you cannot do a lot that you can do with this forum (vbulletin). Blocking ISP's is definitely one way (not sure if you can do that with phpBB) but the other way is to require each user to register with a unique email and have them verify their registration via email. This cuts down on spam tremendously because they can't post until they have verified (clicked) the link in the email. Of course nothing is 100% fool proof but it sure helps a lot.
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Don't put the cart before the horse. Plan your website, thencreate it.
My old clan site got attacked by 'mature content' ads, there is really no way to block them as they will have lots of IP adressess, multiple.
You have two options, make registration harder so research an anti-spam plug-in there's quite a few about.
Or reinstall your board, I was discussing it on another forum and it was proven to work. Backing up your database and leaving it for a week. Spammers try to get in but obviously its not there and they don't come back unless a human user sells your site again.