Disallowing names shouldn't affect existing members:
Please note that you will not be allowed to specify any username that has already been registered. You must first delete that name then disallow it.
It depends how often you'll be adding members. Disallowing names should work fine if you're going to add few users every once in a while.

Or may be you could just password-protect the registration page if it's possible.

I also noticed the "Enable account activation" feature in General Admin > Configuration. Select the value "Admin". That might be an alternative.