Do you have spam-o-matic ban usernames enabled? I think it's disabled by default because of all the registration problems it causes with false positives.
ie - they can get a new IP faster than you can ban them
enabling ban username should shut it down for this username - but it will be a major hassle for other new users trying to register
Best thing would be to enable all new registrations to be approved by mod for a few days - and wait it out - maybe he'll get bored
and ofc be sure to also install good timer-based-registration mod like Spambot Stopper
https://vborg.vbsupport.ru/showthread.php?t=294633