The way that VBulletinboard softwar works, users can delete their posts but not their threads (and not the first post on a thread). I think the reason for this is that an individual post is regarded as being the posters own personal property, but a thread belongs to everyone. (Somethiong like that. )
As far as I know, the only way to delete a thread is to PM a moderator and ask that person to do it.