I while back Paul posted a way to get the IP address. After that how you use it is less simple. I suppose it depends if you allow user sign-up and posting without validation.
I suppose you could gather the IP address of any post then add code which returns a special message if the IP address matches some value.
Maybe the better way is to use server settings to block the range of IP address, that I do not know how to do.
here is Paul's post regarding the IP address.
http://www.gossamer-threads.com/...i?post=238566#238566