He'll find out sooner or later when you sent him to Coventry. Also using miserable users is no guarantee as he will find out at some point and re-register.
Have you tried installing this?
https://vborg.vbsupport.ru/showthrea...multiple+login
It works for vBulletin 3.7.3. Gives you at least a tool to track him down as soon as possible. Be it that he doesn't delete his cookies all the time.