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the only problem with that is, unless the user has a static IP then the ban won't stop him.. if you ban the full ip range of his isp, then u may inadvertently stop any other users that also use that isp from using your site too..
even if he has a fixed IP, he could also still go via a proxy aswell which would get round the ip ban completely.. the only other way is to ban the users email, and set to admin approve or moderate (queue them b4 they posted) new users posts for a week or so..
it all depends on the knowledge the user has..