I mean use cpanels deny ip management /frontend/x/denyip/index.html to ban him entirely....he won't be able to access anything at all.....
There are also hacks from vbulltin.org that will help in the prevention of duplicate names for the same ip.
I think you can do this through the vbulletin cpanel. Copy his ip, go to vbulletin > vbulletin options > User Banning Options > Banned IP Address > *past ip address*