Password protection on my Wordpress page does not work on my MEMBERSHIP Registration Page

I'm using Membership for a Boy Scout Troop site. I set up "password protection" on Membership's Registration Page... because I don't want to allow just ANY site visitor to register to be a member of our Troop's website. However, it looks like Wordpress' password protection is not "strong enough" to suppress the page and the registration options appear anyways. I'm just looking for a way to restrict access to the Registration Page so that strangers do not apply.

Do you have any suggestions ?