If you have access to the IIS server you can block IP addresses from within IIS, which is the easiest to do.  The other option is to block access for the offending IP using your firewall, but in most corporations that is not an easy thing to do.  There might be a DNN 3rd party module to do this, but if there is I am not aware of it.

It's not difficult to write a skinobject to do it. Heres a function to detect the users IP then you can either manually match the IP to an XML table, DNN List, or DB Table. If you get a match then to a response.redirect("http://GoToHell.com").  Just insert the skinobject at the top of your skin.

Hopefully this makes some sense.

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Private Function GetIPAddress() As String
  Dim sIPAddress As String
  sIPAddress = Request.ServerVariables("HTTP_X_FORWARDED_FOR")
  If sIPAddress = "" Then
    sIPAddress = Request.ServerVariables("REMOTE_ADDR")
  End If
  Return sIPAddress
End Function

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