@julian I wouldn't want to give people a false sense of hope that banning the IP will solve the Admins problem. It won't. It would be more beneficial to be able to completely hide a user so their posts can't be read. So only Admins and Mods can see their posts, and delete if necessary. Banning an IP is a poor solution that doesn't work. I can change my IP within about 18 seconds. So that wouldn't stop anyone who was determined to be "a troll" for lack of a better term. If they think they're being ignored. They go away. Win Win.
Perhaps if they're on some form of hellban list, forbid them from making new accounts with some false error of "you already have an account here". But if the IP changed, none of that would work either way. The worst thing you can do is tell someone they're banned, for some reason it makes them want to join more.