This is already more-or-less present in the current version. In the post data section, you have:
Ban: User | IP | Site IP
Disable: User
The first two are visible to moderators; administrators can see all of them. The "Site IP" one bans their IP from the entire GForum installation (Admin -> Users -> Bans), and the Disable one disables the user entirely. Unfortunately, there is no way to prevent them from signing up for another account - but a ban on the user obviously can't prevent this (but the Site IP ban can).
That's certainly an interesting idea; I'll add it to the "good idea" list.
Jason Rhinelander
Gossamer Threads
jason@gossamer-threads.com