I don't know if this is the best solution necessarily but I have a vBulletin 4 forum where I needed to do this.
What I did was create a usergroup for each user with that user as the only member of that group, e.g., usergroup was named "Doe" and the username was "John Doe". Then in permissions for the forum, the only usergroups allowed access were Admin and "John Doe". It's a bit clumsy probably but it worked: no one could even see the forum except "John Doe" and me.