banning an entire ISP isn't such a hot idea, either. Unless you plan to ban all of AOL, or all of (fill in the name of a huge isp here) and then alienating all of these potential users.
Same with email addresses.
You'll never prevent spam posts entirely, there is simply so much money to be made in comment spam that someone will always be willing to find a way to spam. The best thing you can do is make it a slow enough process posting to your board that the spammer will not make any money at it, and will move on to an easier mark.
Disable guest posting. Require registration. Require email address verification. Temporarily turn on registration moderation. Turn it on and off now and then. Especially during off hours or hours that you're getting spammed most. Make it tough for an automated bot to have a run of your board. None of these spammers are actually sitting there posting. There is no money in that. These are bots that are earning someone money while they are off doing something else. If you can break their little automated patters you can prevent most of it.
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