multiple accounts from the same man - IP logs

[Dr_Michael]

Someone comes to my forum registers every day 3-4 accounts and then he just posts advertisement posts for some products and then he disappears. The other day he returns with other nickname, he says the same things: Product A is perfect, Product B is crap and then disappears again. What should I do?
I saw that IP addresses are equal. They all start from 213.5.32.** So it means they are from the same ISP and from the same LAN possibly? I contacted the ISP and they asks for logs in order to act. Where to look for the logs? Which table of database and which fields to search for? Thanx in advance!