As an aside, I wouldn't store the hostname of each and every visitor at all.
1) Doing a lookup of the hostname takes time, and we're not talking microseconds here, it can be quite costly if the route to the visitor is long. This means it will take longer for the site to load on their end, which is a bad thing.
2) You don't really need them as hostnames rarely, if ever, change, so there is not really a need to store them. If at any point in time you want to know someone's host name you can just do a reverse lookup from their IP then and there.
3) Some spammers have their reverse DNS set to "localhost". So if you do a reverse lookup and then ban the hostname you're effectively letting the server ban itself, so php can't connect to mysql anymore, you can't send or receive mail anymore, etc, etc. Obviously this is a Bad Idea (tm).