Requiring a password to register is not something that has even been requested in 5 years, and there have been hundreds of requests over the years, so it's not something that has even been considered.
I would suggest if you want/require this level of security that you actually password protect the folder the forum resides in. Your web host should be able to do this for you. If your web host can not do this then you can always host your site or forum with us and we can place a username and password on the directory containing the forum. Prices are available at
www.webwiz.net Another simpler, cheaper, option is do as I suggested in my last post, but your forum admin can do more checks before activating the account like sending the person an email asking them to conform who they are and giving the password.
You can also use the forum permission system to restrict access to forums so only registered users can access forums so unwanted users can not browse the forum.