After a bit of research and frustration, I've determined that it is not possible to delete a user in this version of PHP-Nuke without corrupting the site and forum database.
I will try to fix the bug in PHP-Nuke, or at least figure out how some way to turn the users entry off in the member list, but until then I can't honor requests to remove a user.
In a worse case scenario, I can disable an account so a user can't log in, but I cannot actually remove the account.