There is no other IP information we can provide, other than what is already provided on the user in the admin CP when you look at that member. You cant get IP information back from emails etc. It would be a case of keep banning them, essentially until they get bored. If someone wants to join when they are banned, and has the knowhow to do so, there is little you can really do to stop them (bar closing registrations of course. People can change IP address (using VPN for example), set up different email addresses and the likes to get around these things.
In my experience, the best course of action is simply to keep banning them, and nothing else. No interaction etc. The more interaction the longer it will take for them to get bored of it