How to stop users registerring certain usernames
 

I know there's an option in vBulletin Options > User Registration Options to ban from users using certain words in their usernames, but is there an option where I can put some usernames on reserve to users can't register them, but that those word's aren't banned from being used in usernames? I.e. Username Johnathan can be registered even if John is reserved.

Do I have to register the usernames to do this?

You could use regex to achieve this, but just registering a user sounds easier. (Well, it depends on how many usernames you want reserved.)