Reverse DNS/IP

Hi guys, Question: I was wondering if anyone knows if it is possible to find the reverse IP/DNS address of a computer please? Reason: I ask this because I wish to produce a 'ban' list of users on a forum who persistantly cause problems, banning specific IPs is an option, but at the moment is not effective because the IP address of one specific user changes, they have a dynamic IP which means they are either using Dial-Up, or specifically paying with their ISP to have their IP refreshed every computer log-in (or Internet box reset). IT's all well and good using $_SERVER["REMOTE_ADDR"]; to help use as a "ban" function, but if the IP keeps moving all the time, it isn't working. So we need to result to a Reverse DNS which doesn't change, it remains the same. Question: Is there a way of finding the Reverse DNS address through PHP please? Thanks for you